depends on what your long term goals are. Do you want to move up to higher stakes? if so, your going to have to think about decisions more and perfect things as your leaks will be uncovered a lot more.
If your goal is to stay in micros/lower stakes, then a more volume orientated mindset is the way to go. Obviously when you play more tables you have less time for decisions, so playing a simplistic game is necessary. There is nothing wrong with it overall as the volume you play makes up for the spots that you miss due to not having as much time.
Finding the optimal number of tables is really just a feel thing. I'm sure you feel comfortable playing 40 tables, but at the end of the day, you are going to be missing a lot of spots, playing really tight throughout, which is fine.
It's a bit trickier when you play MTT, the games go off when they are scheduled, some days you may just bust a heap quickly. So at times you will have 15-20 going, and ohters only a handful. The key for me is that when you have a deepish run going, start to knock back the amount of tables you are loading, this helps you put more energy into the one you have a good shot in.
Long story short, do what you are comfortable with. You have played enough games now that you kind of know the areas you are most comfortable (assuming 12-20 for you). As for finding out how profitble you are in MTT, it's a tough one as the games differ so much, there is so much variance. I honestly wouldn't bother with it and just look at yourself and your game objectively to find the right answer for you. You can also look over tournaments played whilst 20+ tabling to see glaring missed spots or mistakes, and then check it when you play only 10 to see if you make similar errors or not.