The downside of many relays is data use/duplication. Your client downloads all the notes from your follows from all the relays, then removes duplicates client side.

Many clients support the outbox model, where you publish a note specifying which relays you post to which allows your followers to find your notes even if they use different relays. See https://mikedilger.com/gossip-model/

How to pick the best relay for you is one of the weaknesses of nostr imo. I generally pick wot relays hosted by someone that I follow, or cheap paid relays because I assume there will be less spam.