How many devices can use a family plan?
I'm looking at converting from Spotify to Apple Music. If I buy a Family Plan, how many devices can each family member use?
As many as they have. Apple ID's have a maximum of 10 devices that can be associated with them. Any devices associated with the Apple ID can then access the Apple Music Family subscription.
Each person keeps their own Apple ID, then do not use your own Apple ID for anything. You invite their Apple IDs and by extension their devices to Family Sharing.