When I was 18, I pledged a fraternity and was on the rowing team. One faithful evening, I ditched the fraternity and chose to go to IHOP with a girl I liked. I was eating a Tbone stead when all my pledge brothers went to jail that night.
Anyhoo...by 19, I started working full time while doing a maximum of 15 hours. Shortly after that, the college picked me up on salary and I was getting comped tuition up to 9 hours per semester...so that's the most I took after that until I finished my MBA....so 15 hours is it for me. At this time, most schools would allow you to take classes for free beyond 12 hours (full time). Those days are over and you are billed per credit hour.
In my opinion, as long as you don't have scheduling issues, it's possible to do more hours. With online courses, maybe you can do 20+ easier, but schedule offering for whatever major you're in is what makes it difficult to take more classes unless you have a really good plan and can combine hard classes with gen ed courses to maximize what you take. All it takes though is one or two labs that require 5+ hours to complete assignments and you can be hurting.