Unfortunately not specifically in a pivot table. Without an example or template of how your data is structured, I'm kind of shooting blind but maybe this will help.
The only real thing you could do is set up some sort of helper column in your source data to count unique numbers and use that in your pivot. If you do go this route, if you insert this in the middle of your pivot range, then you wont have to modify it, but if you add it to a column outside of your pivot range, you'll have to make sure you adjust your pivot source. (And don't forget to refresh.) For example, if column A is your invoice numbers, you can check if they're unique with the formulas in column B.
Sheet1
<table style="font-family:Calibri,Arial; font-size:11pt; background-color:#ffffff; padding-left:2pt; padding-right:2pt; " border="1" cellpadding="0" cellspacing="0"> <colgroup><col style="font-weight:bold; width:30px; "><col style="width:64px;"><col style="width:64px;"></colgroup><tbody><tr style="background-color:#cacaca; text-align:center; font-weight:bold; font-size:8pt; "><td>
</td><td>A</td><td>B</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">1</td><td style="text-align:center; ">Invoice</td><td style="text-align:center; ">Unique?</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">2</td><td style="text-align:center; ">9944</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">3</td><td style="text-align:center; ">9709</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">4</td><td style="text-align:center; ">9726</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">5</td><td style="text-align:center; ">9839</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">6</td><td style="text-align:center; ">9726</td><td style="text-align:center; ">0</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">7</td><td style="text-align:center; ">9755</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">8</td><td style="text-align:center; ">9623</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">9</td><td style="text-align:center; ">9496</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">10</td><td style="text-align:center; ">9709</td><td style="text-align:center; ">0</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">11</td><td style="text-align:center; ">9089</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">12</td><td style="text-align:center; ">9393</td><td style="text-align:center; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">13</td><td style="text-align:center; ">9130</td><td style="text-align:center; ">1</td></tr></tbody></table>
<table style="font-family:Arial; font-size:10pt; border-style: groove ;border-color:#00ff00;background-color:#fffcf9; color:#000000; "><tbody><tr><td>Spreadsheet Formulas</td></tr><tr><td><table style="font-family:Arial; font-size:9pt;" border="1" cellpadding="2" cellspacing="0"><tbody><tr style="background-color:#cacaca; font-size:10pt;"><td>Cell</td><td>Formula</td></tr><tr><td>B2</td><td>=IF(COUNTIF($A$1:A2,A2)<>1,0,1)</td></tr><tr><td>B3</td><td>=IF(COUNTIF($A$1:A3,A3)<>1,0,1)</td></tr><tr><td>B4</td><td>=IF(COUNTIF($A$1:A4,A4)<>1,0,1)</td></tr><tr><td>B5</td><td>=IF(COUNTIF($A$1:A5,A5)<>1,0,1)</td></tr><tr><td>B6</td><td>=IF(COUNTIF($A$1:A6,A6)<>1,0,1)</td></tr><tr><td>B7</td><td>=IF(COUNTIF($A$1:A7,A7)<>1,0,1)</td></tr><tr><td>B8</td><td>=IF(COUNTIF($A$1:A8,A8)<>1,0,1)</td></tr><tr><td>B9</td><td>=IF(COUNTIF($A$1:A9,A9)<>1,0,1)</td></tr><tr><td>B10</td><td>=IF(COUNTIF($A$1:A10,A10)<>1,0,1)</td></tr><tr><td>B11</td><td>=IF(COUNTIF($A$1:A11,A11)<>1,0,1)</td></tr><tr><td>B12</td><td>=IF(COUNTIF($A$1:A12,A12)<>1,0,1)</td></tr><tr><td>B13</td><td>=IF(COUNTIF($A$1:A13,A13)<>1,0,1)</td></tr></tbody></table></td></tr></tbody></table>
The formula checks to see if the invoice number is anywhere above it, if it is unique, it gives it a 1 value. If it's a duplicate, then it gives it a 0 value. When we then use the sum option in the pivot field, that will give us a count of unique numbers which would look like this:
Sheet4
<table style="font-family:Calibri,Arial; font-size:11pt; background-color:#ffffff; padding-left:2pt; padding-right:2pt; " border="1" cellpadding="0" cellspacing="0"> <colgroup><col style="font-weight:bold; width:30px; "><col style="width:91px;"><col style="width:109px;"><col style="width:107px;"></colgroup><tbody><tr style="background-color:#cacaca; text-align:center; font-weight:bold; font-size:8pt; "><td>
</td><td>A</td><td>B</td><td>C</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">3</td><td>
</td><td>Values</td><td>
</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">4</td><td>Row Labels</td><td>Count of Invoice</td><td>Sum of Unique?</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">5</td><td style="text-align:left; ">9089</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">6</td><td style="text-align:left; ">9130</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">7</td><td style="text-align:left; ">9393</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">8</td><td style="text-align:left; ">9496</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">9</td><td style="text-align:left; ">9623</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">10</td><td style="text-align:left; ">9709</td><td style="text-align:right; ">2</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">11</td><td style="text-align:left; ">9726</td><td style="text-align:right; ">2</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">12</td><td style="text-align:left; ">9755</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">13</td><td style="text-align:left; ">9839</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">14</td><td style="text-align:left; ">9920</td><td style="text-align:right; ">1</td><td style="text-align:right; ">1</td></tr><tr style="height:18px ;"><td style="font-size:8pt; background-color:#cacaca; text-align:center; ">15</td><td style="text-align:left; ">Grand Total</td><td style="text-align:right; ">12</td><td style="text-align:right; ">10</td></tr></tbody></table>