Usually it's slot number that restricts the amount of debuffs you can use. Level 7 debuff lapis take up 2 slots, but they have MUCH higher chances of activating than level 6 debuffs. The chances of proccing a level 6 debuff are not really dependable. If you manage to get like 7 attacks off, then yeah maybe you can count on a level 6 to proc, but by that point, the enemy should be dead.
Thus, the reason why you probably only see 2 debuffs, is because many people run 2 level 7 debuffs. It's not like there is any restriction on the number of debuffs you can have though. So you CAN technically stick on Pain6, blindside 6, slumber 6, confusion 6 or something like that, but the proc chances will be really low for all of them. It's much more dependable to rely on two level 7 debuffs, which is probably why you see only 2 so often.