John probably averaged them with a single range, like
AVERAGE(A1:A50000). If you are trying to enter each cell in the
formula individually, like AVERAGE(A1,A4,A6,A10), then the limit is 30
in a formula. If that is the case, try to come up with a different way
to do this, perhaps using ranges. There can be blank cells or text in
the range that won't affect your calculation, so you can run a range
across some things, just not other numbers.