how many kings of snakeheads are there

All depends on the species you have. I've seen a cobra SH housed with a silver aro, oscar, paroon shark, and various large catfishes. The sh was the smallest of all tankmates.
But a Red or Giant SH grows very large that it probably kill every possible tankmate.

Generaly snake head plus tankmates over time equals bigger snakehead. They will all eat anything they can swallow. The last one I saw returned at the LFS tried to eat his owner's hand. Talk about biting the proverbial hand.

They are not as bad as most people make out, you can get small SH's reaching a max size of 12" to 14". If you keep them well fed and don't feed any live foods (meaning feeders, that tends to make them aggressive towards tankmates) to them from an early age you should not have a problem with tank mates. That being said there are always exceptions to the rule so just to be sure, make sure the tank mates are not bit size.

Sorry in my last post I meant "bite size" not "bit size".

There are a few SH's that you can put in a 55g, check this site out, Click here

Bout 15 years ago, i raised a Red snakehead (Channa micropeltes).
I got him when he was about 1 inch long.....a massive eatter even then..
Over a 7 year period he grew to be about 2 feet long, and the only thing swimming in my 135 gallon tank, besides a 10 inch Plako....
Crazyest fish i ever had....
Total personality.... i fed him 30-35 feeders at a pop...and he'd eat em all within seconds... even bit me once while trying to drop the feeders in...

At the time I had 5 tanks running...
Over time, and with enough moving, i got rid of all my tanks...Including the Red Monster....

Now, 10 years later, i find myself this week with a 55 gallon tank a neighbor handed me....

Here it goes again...