There are no realms in War Mode, just RP War Mode shards and normal War Mode shards - which are in theory faction balanced per BfA zone. Join an RP realm to be in RP WM, and a normal realm to be in normal WM.
They say RP War Mode is faction balanced in Alliance favour, and normal War Mode faction balanced in Horde favour. But, only thing I’ve noticed is… RP War Mode has a lower volume of players in zones than normal War Mode. I prefer normal WM.
The realm you choose could depend on if you want to be in a WPvP guild - check this one for Horde!