I'm not sure if either I'm not understanding the complete function of count or if it is busted. What I try is to check to see if there are more than 1 player on the server or in the case of an area. I couldn't get this to work properly when checking an area
testfor @a[x=72,7=64,z=80,r=3,c=2]
so I tested it for all on the server.
If I have it set to testfor count=2
testfor @a[c=2]
when I'm the only player on, the comparator still outputs a signal. I would expect this to NOT send out a signal as the players on is 1, not 2.
If you want to test for more than one player on the server, you should be able to do
which will output the # of players online.