Two per gem, and the are the same strength as if you were doing a single enchantment. I have so many freaking soul gems now it's ridiculous. If you want to get a lot of them just go through dwemer ruins, they are everywhere, think maybe the dwemer automatons were powered by them or something.
That's exactly what it is. They are powered by captured souls, which is why if you try to use soul trap on them, it fails. They don't have a soul to capture as it is already in a gem. I have a bunch too. Much like higher end arrows, I tend to try and never use the higher end gems. My current bow, the Nightengale one, uses three commons to fully recharge if it is totally empty. Once I make my final armor set, I'll probably not be so shy of using up the higher end ones as I'll know I don't need to save them to enchant armor anymore. My thought going in though was that I would need two for everything since it was a dual enchantment. So I was trying to save at least 20 to do all the armor, shield, bow, amulet, ring, and at least one sword. Now I know I can halve that.