Lol lots more than 6 fish can be put into a FH
Mostly you would get Midas (considered better in color than a Red Devil, but both are used) and Trimac mixed with a lot of other things, commonly Redheads. However "Flowerhorn" is a veeerrry broad term, seeing as many of the Asian breeders use different fish to make the FH (FLowerhorns are much more highly valued in a lot of Asian countries, good luck charms i guess?)
So basically anything can be ised in making a Flowerhorn as it is basically another word for hybrid at this point. Maybe when it was new and had a specific recipe of fish to produce them you would be stuck with a limited list, but not anymore. There are many different kinds of FHs and many things to go into them.
Things like Blood Parrots, however, have less variation in the species used to make them. There are more than 2 im sure, just because there is a variation in color and body shape that, IMO, is slightly larger than most species. But it is a very unique shape that probably only has a small amount it can change before it loses that.
Also, Festae can breed with pretty much all CA. So yes they could be in the making of FH but ive never heard of them being used commonly, they are a relatively uncommon, large, slow-growing, and aggressive fish which doesnt make them practical to mass produce either as themselves or to put into Flowerhorns.