Hoe many tax exemptions should I claim?

I currently am head of household, with a Wife and 2 children. My Wife also works part time. I currently am claiming 0 federal tax exemptions and my wife is claiming 1. I'm trying to increase my take home pay, without owing the IRS. Should I adjust this, and is there anyway to see how much of a difference it will make? Also, my Wife is pregnant with twins. Should I make any additional adjustments after they are born?