Questions about minions for Demonology

Hey, I’m new to WoW and I have some questions about the demonology spec. I’m playing Wrath of the Lich King.
1) how do I get soul shards? Do I only get them from the Drain Soul spell?
2) What is the max amount of minions you can have at a time?
3) I know that there are temporary minions such as imps (from hand of gul’dan) and Fel stalkers, are there any more?

1. From drain soul spell only, i'm not sure if there are others.
2. At the time > only 1
3. Voidwalker, felguard is one of the best for demonology.