FCPCEO wrote:
I am trying to create a spreadsheet in Excel that collects golf scores and
averages them out. How do I set a maximum of 20 entries with the 21st entry
causing the first entry to drop off automatically?

I suggest you rethink your approach:

Do not limit the number of entries, but adjust your analysis to examine
the 20 most recent entries. Google "excel rolling average" for more ideas.