Sure, you can power as many as you like. You'll need a larger capacitor if you're using the DC-DC versions with a bridge and a filter cap. If you were using a 330uf cap for one, just increase it by roughly 330uf for each added converter.
Remember, most of them have a common ground with one side of the AC, so you just have to make sure you wire accordingly. Each side of the AC feed should go to the same side of the converters. The input and output ground sides are usually common.