i honestly think it's hard to predict the number of side characters you'll need before you start plotting those 3 parts needed. it might be easier to think of the story divided into arcs between the MC's childhood, adolescence, and adulthood and feature a few prominent players for the respective sections.
you can have as many unnamed background characters as you want to populate the world and generate "noise," but you'll want to only focus on a few of them as your side characters to create "sound," if that makes sense.
i'm writing a mini-arc now and my side character cast consists of 3 main side characters, 2 less important ones, and 1-2 more named minor characters that have relationships with the side characters but not so much with the MC. the rest are all "mob A" or "villager B" types.
depending on how much time in the spotlight you give them 3-5 side characters might be all you have time for. the more names/side characters there are, the more the readers have to keep track of, and sometimes that's just not worth it if these people don't ever appear in the story again. if you're going to give them a whole identity and backstory, i suggest you better make sure they're worth remembering. otherwise, "the janitor" or "the shopkeeper" are good enough as stand-ins.
and if you mean side characters as, idk, side flings or whatever, i've got no advice there except to make their descriptions pretty i guess? hahaha.