https://wccftech.com/wow-classic-account-character-limits/

Blizzard has provided additional details on the Wow Classic account character limits.

Community manager Kaivax provided clarification on the character limits in World of Warcraft’s vanilla server option following a question on the official WoW forums. Interesting to note is that there won’t be an overlap with the character limits for retail (i.e. Battle for Azeroth) World of Warcraft accounts. Players will be able to have up to 50 Battle for Azeroth characters and up to 50 WoW Classic characters at the same time.