There are no limits for how many characters of either sort that you create. While you are the sole owner of your PC (playable character) an NPC becomes public property and anyone can play with or use the NPC.

Playable Characters vs Non-Playable Characters

Playable Characters	Non-Playable Characters
Can Earn Levels	Cannot Earn Levels
Can Earn Coins	Cannot Earn Coins
Can use items and engage with the game directly.	Cannot use items or play the game directly.
Can participate in prompts and events.	Can come along as a supporting character.

Playable Monsters vs Non-Playable Monsters

Playable Monsters	Non-Playable Monsters
Must belong to a PC.	Can belong to anyone or no one.
Has to be earned through gameplay.	Can be whatever.
Earn levels through gameplay.	Has no set level.
Can be traded and bred.	Cannot be traded and bred.