Two is the usual answer
For most students between eight and sixteen, two lessons a week is where the curve is steepest, and it stays steepest there for about a term. That is the default and it is the default for nearly everybody.
Three is where it flattens
Not because the third lesson is wasted, but because it eats the hours the student needed in order to try things alone and get them wrong. Getting things wrong unsupervised is most of how the learning happens.
The exception is a deadline
Six weeks out from a test, three or four a week is fine, because the work has changed from learning to rehearsing and rehearsal does not need the same recovery time.
The tell to watch for
If a student arrives having done nothing since the last lesson, the answer is fewer lessons rather than more. Adding a fourth to a student who is not working between the first three buys nothing at all.