There is no set limit for the total mix although I think 20% is pretty close to average for many mixes, it can be higher or lower. Really depends on the flavorings and how many are being used, some are stronger than others. There's many recipes floating around where you can see it go both ways. I personally have a 10% limit for total flavorings in the mix but that's just me.
I would recommend starting low and working your way up, it's much easier to add than take away and many flavorings can have an off effect or even a weak effect if you add to much.