I need to reload one of my guns but whenever I put ammo in one of the characters slots, it only adds a single bullet. So how do you attach more than on bullet to a character?

Well if you're meaning in story mode it just means you need to collect more ammo from side-objectives like collecting missions, if you mean online it's because you have used most of your ammo and you only have the ammo already in and one other bullet. Sorry if I'm being too vague