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.
This is starting get a little bit off topic but hopefully I can try to clarify the AdSense stuff.
AdSense allows up to 3 AdSense for content units, up to 3 link units and up to 2 search boxes per page - see https://support.google.com/adsense/answer/1346295?hl=en#Ad_limit_per_page for the actual policy wording.
MoonBit.co.in has 4 adsense units because it is a "premium" adsense publisher account, for which there is written authority from the adsense account manager to have the extra 4th unit. (do a google search for "adsense premium publisher program" for lots of articles about this)
The other ad units that look like AdSense are actually through EPOM which serves adverts from the google system and are allowed up to 4 per page alongside the AdSense units.
I think it's ok and I'm happy that this platform is here ( after Bitmedia.io has started scamming ).