Can you put fifteen digits in excel?

Answer 1

Yes

you can enter a number as text in which case you can enter I believe up to 256 digits.

or you can enter 15 single digits numbers into 15 different cells

or you can enter a single 15 digit number in a single cell (this is the physical limit set by the software on how many digits a number can be allowed to have) though it will most likely be displayed in scientific notation unless you format it differently.