2: Eliminate redundancies. You will also find some Secondary Characters can be combined. For example, in LADY, I had two sidekicks to Tom Flynn, one an ally and one a foil. I cut it to one, Jeremy, who could serve as both.
3: Don’t introduce your secondary characters all at once. It’s way too confusing. Space them out. Each time you introduce one, put [NSC] (New Secondary Character) in brackets in your manuscript. As you get toward the middle-ish, or even the end of writing your draft, do a SEARCH for them. Are there too many, too close together?
4: Help your reader keep them straight. A few good ways
(1) Distinguish the voices. In Stef Penney’s TENDERNESS OF WOLVES, she has four or five different narrators. And the voices are so different you don’t even need to be told who’s speaking. This difference in voices will often evolve naturally if you’ve done your off-stage work.
(2) Associate characters with a particular hobby or place. Someone who always is fiddling with his stamp collection or playing her piano. Someone who goes to the same bar every day after work.
(3) If it’s been a while since we’ve since the SC, remind the reader with a bit of dialog or internalization of who she is.
(4) Make sure your character names don’t sound too similar. Blackwell and Boulter and Bingley and Burns … change it up.