The Sentry mix 2 fan controller uses 30watts per channel, with 6 channels adding to a total of 180w total watts for the device. This means that you are able to use up able to use certain number of fans that equal up to 30watts per channel.
Lets take into example you wanted to use 6 NZXT FZ 120mm fans on one fan channel. Each fan consumes 1.68 W normally and 3.60 W max. I want to make sure that if the max is hit, that the controller is safe. So 6 x 3.6 = 21.6 total watts and that is safe to do.
If you wanted that many fans on a channel, you need to use a 3 pin fan splitter. You will be able to find them at most online retailers and specialty shops.