Warlock Trick: Have as many minions as you want ( 5 Minutes )

First: Queue for a dungeon with a friend.

Second: After the dungeon is over, Have your friend stay in group still, make sure he doesnt leave.
Third: Enslave a demon, And click "teleport into dungeon" ( You will notice your enslaved minion isn't there in the dungeon with you )

Fourth: Teleport out of dungeon, and then you will see your enslaved minion next to you, however you will not be able to dismiss him what so ever, And he has no cast bar but the enslave demon buff is still on him for the remaining 5 minutes.

Fifth: Enslave another demon, and do steps 3 and 4 over again.
Keep doing this as a rotation to get more and more minions.

Edit: A few more details:

1) You need to have a pet out while casting enslave demon, otherwise the enslaved demon will just vanish when you teleport to the outside world.
2) Hearthstoning will dismiss all the extra demons and turn the last demon you enslaved hostile. This happens even if you are on the same continent.

Currently trying to find a decent place around Orgrimmar that has a lot of enslavable demons.