What’s the highest amount of characters you can have at any given time in the campaign groups here, including and excluding deactivated characters?

I believe that up to 12 different accounts can join a campaign and each can have 99 characters created there.  I don't know if there is a cap on assigned/active characters or not.  

I think if nobody in the group I’m arranging drops out, 12 will be the exact amount of players

I got up to 6 lv15 players. The DM was not amused