With the leveling changes made to World of Warcraft: Shadowlands, the developers announced on Twitter the lifting of the restriction on the number of Demon Hunters per server.
Currently and since the deployment of the Demon Hunter in Legion, it is only possible to create one per server, in the same way as for the Death Knight in Wrath of the Lich King. Starting with the Shadowlands expansion, these two classes should start at level 8, so with a much less significant advance compared to the non-heroic classes.