There is no correct answer, but try 5. Three Saturday starting at 11:00 AM or noon depending on how long of a drive you expect people to undertake. two on Sunday to allow people to get home. No race start after 3 PM.
You can have more races but that will mean shortening each race or if you are fortunate enough, starting directly in the vicinity of the launch area.
Many years ago after participating in one day college type sailing with 10 races in a day, I was disappointed with attending weekend CBYRA type regattas that only had a total of three races. If you schedule more short races there will be a chance for a larger number of people to do well in at least one race and thus feel good after all the races are over.
Go for more rather than less.
FS 3512

I agree with the previous post. In general if it is a two day regatta go for four races on Saturday and two on Sunday. I like to have a throw out if we hit six races.

If it is a one day regatta, depending on your venue and what you want to do, you could absolutely do six races in one day. At our fall regatta we did six races in four hours. It was so cold that people could not take anything longer.

Never have just one race on the second day. People do not like to return for just one race if they live close enough to drive but not close enough to make the drive convenient.

Good Luck!