Kaivax posted today replying to another user on the WoW Classic Battle.net forums today.
In WoW Classic, you’ll have the following character limits per WoW account:
A maximum of 10 characters per WoW Classic realm
A maximum of 50 characters across all of the WoW Classic realms in your region
Only one faction per realm on PvP realms
No overlap or regard with the character limits in Battle for Azeroth
You’ll be able to have up to 50 Battle for Azeroth characters and up to 50 WoW Classic characters at the same time.