There is no fixed number, but for a major competitive exam, a full-length mock taken regularly (often weekly, rising to a few per week near the exam) is a good guide. What matters far more than the count is analysing each mock properly, an un-analysed mock is almost wasted.
Start mocks once your syllabus is reasonably covered. Early on, do topic-wise tests; once most of the syllabus is done, shift to full-length mocks under real timing.
Increase frequency as the exam nears. Many aspirants do one mock a week initially, then two or three a week in the final month to build stamina, speed and exam temperament.
Analysis is the real work. After every mock, spend as much time reviewing it as taking it: which questions you got wrong and why, where you lost time, silly mistakes, and weak topics. Then revise those before the next mock.
Example: A NEET aspirant in Hyderabad took a full mock every Sunday for months, then daily mocks in the last two weeks, but crucially logged every mistake and revised it. Her score climbed mainly because of the analysis, not just the number of tests.
Do not over-test without studying. Endless mocks without revising between them just repeat the same mistakes. Mocks diagnose; your revision cures.
One practical tip: Keep an error log, a notebook of every mistake from every mock and why it happened. Revising this log weekly fixes recurring errors faster than simply taking more tests.