I believe in one life.
(please y'all, don't take offense to this, as I do not mean it to be offensive in any way toward anyone, this is just my opinion)
I think that the consept of reincarniation is a wishfull way of thinking so that when we screw up we can chuck it aside and say "oh well, I'll try and do it right next time". I think it's often used as a scapegoat to not have to take responsibility for our actions and to put our minds at ease so we can sleep better at night and not have to "fear the angry, vengfull, judgmental God and His wrath". Basically, IMO, it's just a way to make ourselves feel better...kind of like chocolate, doesn't solve the problem but tastes good at the time.