Blizzard has announced that players may have ten characters per realm in World of Warcraft: Classic. However, players can have up to fifty characters spread across all of the Classic realms in a player's region. Players may also have characters of only one faction on PvP realms and there will be "no overlap or regard with the character limits in Battle for Azeroth". This means that a player can have up to fifty characters in Classic and fifty more characters in BfA though it sounds exhausting!
Check out the discussion on the World of Warcraft Classic forum.
