Got these old ice cream tubs from a local ice cream place. $1 for a dozen. I wanted to increase my rainwater storage (currently have 2 rainbarrels). Realized I could stack these guys up as much as I needed. A few drilled holes and a spout off a cracked kombucha kit and bam. Homemade rainbarrel.
This is part of a set of daisey-chained barrels so I had to keep it to 3 tubs for height reasons. All told it only adds about 8 more gallons of storage but every bit helps. You could stack as many as you wanted though, within reason.
I kept it simple but you could also add additional sealing between the bottom-lid connection to limit loss that way. I will add a few extra pictures in the comments.
There is no maximum here. I think thats a US thing.
Maybe some specific state, county or city, but definitely not a general rule in the US.
Most of the Western USA has some form of ordinance for it, but it applies to other arid parts of the world as well.
I've only ever lived in Oregon and Washington, in both those states its legal if you are just doing it off existing rooftops and using it on your property. I think, in both those states you could run into issues (or at least need a permit and water rights) if you build a system specifically for catching water.
I think Colorado is the only state with the 110gal rule.