In George R.R. Martin's A Song of Ice and Fire, I can't keep track of all the Freys. And I'm very glad that there's a character list in the back, especially when there no fewer than three characters named Brandon Stark. (Only one alive at the time of the story, but the others are referred to.)
If you have a lot of characters, consider a Dramatis Personae list at the novel's end.
Certainly, if your reader can't keep track of your characters, you have a problem. But there's also another limit on your characters: you should have no more than you need. How many is that? It depends on your plot, of course, but it's easy to multiply characters by creating a new one every time you need someone for a plot development. Instead, think about the characters you already have, and try to make one of them do the job. You may have to jiggle the character's personality or history or position in the story, but this also has the advantage of making the character a little richer and deeper, if they're not just used once and thrown away.