Each campaign must sell at least 10 shirts in order for them to be printed and shipped. At 20 shirts the campaign will become profitable (before 20 shirts, we're just covering our production costs)!
However, the more tees you sell, the more profit you make. So you'll want to shoot for specific goals (36, 50, 200, etc...) which ultimately give you more profit per item sold! If someone buys a shirt but the total sold doesn't reach the 10 shirt minimum needed to print, their card will never be charged.