I don't know that it has a plant limit per se, you can do a scrog/sog setup with many smaller plants or something with one or a few larger..
My understanding is it is meant to offer coverage in up to a 4x4 tent for bloom, so whatever you can fit comfortably and manage keeping the environment where it needs to be for them.
I have used it on as few as four plants in 5 gallon pots or as many as 26 in 1 gallon pots (currently, started as 32), from my runs I would say less is much easier to maintain the environment but I guess that's a no brainer..