You get to keep all of them so they accumulate, so if you prestige and then prestige again then you will end up having 2, if you prestige 2 more times then you will have 4, and so on. I'm 10th prestige and have waited for awhile to use them and they were all still there to permanently unlock guns, perks, or scorestreaks. If you have questions about my answer then feel free to ask away and I'll be happy to clarify. There should be no worry about prestiging over and over again though.
EDIT: I just hit master prestige and wanted to mention that if you don't have 10 classes by the time that you hit master prestige you'll get locked out. I'm stuck with 9 classes because I didn't spend my tokens on extra classes. It's not a big deal, but I thought it was worth mentioning for all the people that really wanted that 10th class.