I assume you mean seeing ### in the cell itself in the sheet, and not in the editing bar?

If so, then the reason you are seeing ### is because the column is not wide enough. There are a few ways to widen the column...
Point the cursor to the right of that column's letter, watch for the cursor to change shape, then click and hold and drag to the right to widen the column
or
double-click on the line between that column and the next
or
right-click into the cell showing ###, look for the Column Width button, click that and select a larger number than what is already here - you may need to play around doing it this way, to get the width you want

I generally use the 2nd or 1st options above

want to be able to type: 20000000+ (8+ digits)
Note that excel only allows 15 digits, after that, it will just show 0's for additional digits