I prefer more natural-looking tanks, so I tend to limit the plant species to no more than perhaps 3 or 4. Sometimes i stay with one genus, sometimes I try to add something different for contrast. If you want more of a plant tank that happens to have fish, there are the "Dutch" style tanks at the opposite end of the spectrum.
Plants have varying requirements respecting light, GH, and nutrients, and depending upon the species this can be sihgnificant. So not all plants will work together. There is also the issue of alleopathy, involving the release of chemicals by some plants that will inhibit other species. Thnis is something that most of us don't know too much about.
I tend to go geographic, so in my Amazonian tanks the plants, like the fish, all occur from that geographic area. This can limit options though, so in other tanks I have plants from SA, SE Asia and even Africa mixed, just to provide variety or deal with specific circumstances like shade, etc.