I don't remember or I forgot there was such limit
"Only one faction per realm on PvP realms"
I was planning on 5 boxing teams on the same realm both alliance and horde but now not sure what to do (probably not double my subscriptions which would be how to achieve it)

They are also limiting to 10 character per realm and 50 total (per wow account I assume and not battlenet)

Source https://us.forums.blizzard.com/en/wo...ssic/163599/23