How many mocks?

The opinions are subjective because each person has a different style that works for them. If you have studied for a standardized test before, you should know what mock test strategy works the best.

If you haven't, I'd recommend that you schedule a mock test the weekend before your actual test, and at least 48 hours before the actual test. So if your test is on a Monday, take a mock test on Saturday at the same time of day as that of the actual test. This way, you'd have time to review the test and time to recover after. Anything more than light studying in the 24 hours before the actual test is likely to cause fatigue which is one of the worst things for such a taxing exam as the GMAT.

On the other hand, some people do take a mock test the day before the actual test and keep studying until the night, and this strategy works for them. I would say these people make up a minority of the population.

If you are asking about general study strategies rather than ones for just before the actual test, I usually advise my students to take a mock test per weekend. Anything beyond this frequency is likely to cause burn-out and compromise the usefulness of the experience. It is quality that matters.