Really, the current number of displayed buffs is fine - SE needs to be better about prioritizing which buffs show up for each player, however. As a SMN, I don't need to see whether or not some DRG is keeping Phlebotomize up. I want to know whether MY DoTs are up, so those need to be visible and, preferably, the FIRST things displayed. NINs don't care about my DoTs, but they care about their own debuffs - and they also care about WAR's Storm's Eye. Pretty much all jobs want to know when something like NIN's Trick Attack is up. I'm not sure how difficult that would be to implement, however; SE would need to set up a different display order priority for pretty much every job, as different jobs care more about other jobs' debuffs in different ways.

It'd be nice if enemy self-buffs were also made more distinct in some way. In the player's case, buffs and debuffs are plainly separated, but for mobs they're all mixed in together. Since some of those buffs are VERY important to keep track of, it'd be nice if they were separated in some fashion. They probably wouldn't need a whole lot of room to do so; it's rare for an enemy to have more than one or two buffs on at any given time.

As for the maximum POSSIBLE capacity for buffs, hidden or visible... If there's ever a case where a SMN's DoTs are not landing because there's no more room, this is plainly unacceptable. A SMN who can't land DoTs may as well not be present at all. If this is something that can happen, SE needs to fix it.