Without using a macro, any value entered into a cell is going to overwrite any formula you put there to limit the range. What you need to do is to use a second cell below the one you want the use to fill in to handle the limiting.
There are lots of ways to do it, but a simple one would be to put something like:
=MIN(A1,20)
in the cell you wanted to contain the value limit to 20, and:
=MAX(0,A1-20)
in the cell you wanted the remainder to go into.
Then any calculations you did would go from those cells rather than the one the value was originally typed into.
Edit: Please note that I've not got Excel handy so I've not checked that MIN/MAX are actually the right function names.
