I think it's 3 per visible part of the page, if the page is scrolling down you can add 4th and so on. Also, I've always wondered how sites like Moonbit.co.in has 6 adsense units and they are obviously paying them?
It's 3 total. Now, there are a couple tricks:
1) You can use multiple accounts (not "allowed," but you can sometimes get away with it until you get caught)
2) Some networks are allowed to use multiple. For example, I work directly with a network that has a special agreement with AdSense, such that I can run 3x AdSense on my account + up to 5 from the network account, for 8 total
Most people just fraudulently (or... break the ToS) in the hopes they won't get caught. As you'll learn from a quick search, some faucets got banned from AdSense due to a lack of content, and others got away with it. It just depends on when the manual review comes into play. You can be guaranteed, however, that when manual review DOES come, anyone running AdSense on their faucets WILL be permanently banned and all funds forfeited. People just hope they can earn enough money to get paid before that occurs.