The world record for most 4 leaf clovers goes to a prisoner who collects them from the prison yard. He has over 72,000 of them.
Four leaf clovers are not that uncommon. For every 10,000 three leaf clover, there's a four leaf clover. It's a simple mutation, which can yield, like was pointed out, up to 18 leaves. I found a five leaf clover once at school. But naturally, the four leaf clover gets all the fame. Most likely because the four leaves mean something.
According to legend, on a four-leaf clover, each leaf stands for something. One leaf symbolizes hope, one is for faith, another for love, and the fourth is, of course, for luck.