To me, it depends. I think a lot of tracking has to do with the writer’s ability to truly differentiate characters. I’m not just talking outward description. I’m talking vocabulary, cadence, voice, background, goals, fears . . . In aggregate, a character will seem alive and memorable or not. I think, in some ways, the better writer you are the more characters you can handle. I make rules for my characters’ speech patterns at the beginning. Sometimes simple like: he will only say five words at a time and never use a word more than two syllables that isn’t a proper noun. Or, this person doesn’t understand consequences so never uses conjunctions. Or, this person fished their whole life so they understand everything via nautical terminology. Usually, by the time I add 3 rules, people start feeling pretty distinct.
There are other easier tricks, I think. Avoiding similar sounding names. Spreading out when you introduce them. (Total length becomes a factor, here, since I don’t think you want to be introducing anyone major from Act 2b on.) I think the “first impression” idea is pretty important. Some of this is on the reader too–some people pay better attention to character, some focus on the plot, some have better memories. And how fast they read. Someone who puts it down for 3 days halfway through might need a refresher.
To directly answer your question, ideally I could write so that people didn’t need reference. But Shakespeare (in his plays) listed all the characters at the beginning. More recently, George Martin had to make family tree for all the houses in his ASOIAF series (GoT).