To be honest, you probably only need one. But if you can afford two, then go for it.
The amp you get will depend on how much power you need/want it to give to your sub(s). Two subs require twice the power of one.
EDIT: Yes, we are both suggesting 4-ohm subs--but for different reasons. He says to get them if you get two subs in order to wire them to 1 ohm. The less ohms an amp sees, the more watts it will put out. But not all amps are able to handle such few ohms. More expensive ones are able to. I am suggesting that you get a 4-ohms sub, because I think you really only need one. And you can wire 1 sub that is DVC 4-ohm in a way that the amp sees two ohms. Basically any amp is stable at 2 ohms, and is more realistic for your price point. I wasnt expecting much when I got my type-r and 500watt amp. But when it was all said and done, I was completely blown away by it. I have the best car audio system out of anyone at my high school.