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!