You should always start by determining the number of positive cells in your least frequent cell population that you want to analyze (or sort) and then back calculate from there to estimate the amount of starting material needed for your assay. So if your rarest population of interest has an expected frequency of e.g. 1 in 1,000 (0.1%) and you want to have 100 positive events in that gate then you need to analyze a minimum of 100,000 total cells.
But note that you probably have lost some cells during your staining protocol and part of the sample volume cannot be analyzed (~100 µL dead volume in the instrument/tubes) so it would be safest to start with at least half a million cells per sample and have a final suspended volume of no less than 200 µL in your FACS tube. And you need the same half a million cells per tube also for your single stain/FMO controls! Furthermore, if you are performing cell sorting take into account that the sorting efficiency can sometimes be as low as 50% as some positive cells “traveling together with others” are intentionally put to the waste tank (depending on your sort settings) to guarantee the purity of the collected population. So you may need to again double the amount of starting material if you have a difficult sample with lots of debris and aggregates.