Since I don't know what strategies other clients use, I can’t give a precise number. The suggestion of 2–4 comes from the NIP.

If you already understand the role of each relay very well, you can keep your current setup, it's really not a big deal. The prompt is mainly for users who had randomly added a bunch of relays without much thought.

The fact that your notifications dropped after you reduced your relay list actually proves the point: most clients won’t send events mentioning you to all of your read relays.