You must do all 4 official UCAT Mock tests. This is a minimum and is free on the UCAT official website.
Additionally, you should be doing two Mock UCAT tests every day in the 1-2 weeks before the actual exam. There are also mini-mocks available (unofficial), which also help improve your score. Finally, go through all the question banks on the official site. As well as just doing the mocks, spend considerable time going over your answers and reflecting accordingly. It is usually a big step going from untimed practice to performing well under timed exam conditions, so give yourself the best chance by doing at least two Mock UCAT tests per day in the final days of your preparation.