Yes, a cell value can have more than 30 decimal places. The constant 1.23456E-60 is a number with 60 zeros after the decimal point followed by 123456. To wit:
But no, we cannot specify a format with more than 30 decimal places. For example, the best we can do is Number with 30 decimal places.
For such numbers, we must use the Scientific format.
Scientific with 14 decimal places with show you 15 significant digits. That is all that Excel formats. But that is not the limit of the precision of cell values, contrary to the article that "footoo" cites. For example, the exact decimal representation of 0.123456 is 0.123455999999999,99629718416827017790637910366058349609375.
(I use period for the decimal place and comma to demarcate the first 15 significant digits.)