I believe that there is no limit. You should simply work, 2-3 hours per day on any language you have chosen, and when it is complete, move on. My list of languages, not including extras, now numbers in 28 languages. Will I learn them all? Probably not! However, I have made an amusing abbreviated goal for myself. RAPID. It stands for Russian, Arabic, Polish, Italian, and Dutch. Some of the most interesting languages(to me) in the world. So, I'd say about 5, or more!