Stickers are given every time you win a Tier. They also sell for a certain amount of money, depending on where you get them in the Championship. There are plenty of them and you can only have 24 until you have to reset them during the prestiging process.
When you prestige, you get a choice of 2 to select from.
And for the two of you who think it does, it does NOT matter which one you chose as it will NOT define whether you gett a super or legendary box.