"I wanted it to add numbers", which numbers?
The formula =IF(G81,1,0) will return either 2 or 0 depending on if the checkbox is checked.
So, = A1 + IF(G81,1,0) will add either 1 or 0 to the value in A1.
Hence the question "Which numbers?"
The IF / checkbox part is done (unless you want to change 1,0 to 1,-2 which the OP implies)
All that's left is specifiying the other numbers.
(BTW, the checkbox is not in I8. It may be covering I8, but its not in the cell. Nor is it interfering with something being in the cell, beyond the issues of a)seeing what is in I8 and b)selecting I8 without clicking the checkbox. The location of a control on the sheet does not create any relationship between a cell and that control.)
I can't think of why all the checkboxes are reacting as one, unless all of them are linked to the same cell.