Its depends on the individual fish, but most are very aggro,sometimes there is the possibility of more than one FH living together if you have a large enough tank, I'd say 6 ft minimum.
THey are very aggresive and territotial. I had one that was 4" in a SA community (this was before MFK was around) and it killed a 3.5" GT, 7"TSN and a 4"JD. It turned out to be a sweet fish though. I had him for 7 years before he died. He was over 12".
Not only the above but for the reason to insure a flowerhorn always looks it's best it really should be kept alone, not with any other flowerhorns or cichlids.
its much easier to list the fish you CANT keep together. an overall majority of big ca/sa cichlids will live in a tank with other species without much problem in the right conditions. all but the very worst offenders can be happily put into a community.
as was said 5x already, flowerhorns are extremely aggressive, and will not tolerate anything in its tank most of the time. they will fight anything that you put in with it to the death eventually, and more oftne than not, the flowerhorn comes out on top.