It's really your call. Whether you're cutting or trying to add muscle, 2 caps will provide better results, but 1 cap still works quite well.
As the poster above mentioned, it can increase appetite. This does not happen to everyone, but it does to most...to varying degrees. However, this effect greatly diminishes after a few weeks. Whether or not this effect is problematic for you can only be determined through experience. I know plenty of people who have used it during contest prep and we're fine...and others who couldn't because of the hunger. This generally isn't too big of a problem when you're still eating a decent amount of food, but if you suddenly cut your cals way down to the point where you would already be very hungry anyway, it will just amplify those cravings. Experience will provide you with the answer.
I do think MK excels when used during mass-building phases...simply because all of its effects are well-suited for programs centered on muscle growth.