Can a writer have too many plot lines and important characters in a novel? If so, how can this problem be avoided?

You can fold “single use” characters into another character or create one from several characters that were “smaller” before.

One of my novels introduces a friend for the protagonist, Jemima. This friend exists to be there during a trial and is discarded (quite accidentally, forgot I had her) after Jemima heads back to Georgia to see her sister. If I wanted to keep Evie, the friend, I would need to expand her and then figure out a role for her after Jemima goes back to Georgia. As it is, Evie can be folded to the side, and if her stuff is rewritten, she’s a “single use” character, not a friend who could be expected to recur.

So, you can analyze the purpose of each supporting character and fold up the ones that work “together” to be one, more effective, character.

Another one of my books has a legal office in it. Anne, a specialized accountant, appears in two scenes. She’s never used as an accountant, she just had to have a reason to be there. Luz also exists in this office space, and is also a source of gossip. I could fold Anne into Luz and lose basically nothing. There’s probably no reason to have two gossipy co-workers that don’t interact together.