I personally try to get my unfinished game to demo status before working on a new one lol. But it's really hard to have an idea and concept convincing enough to oneself to keep working on it for a very long period. I know quite a few people who have about 10 game ideas and are working on 3 of them at almost the same time, I guess it really depends.
I used to work on multiple projects at a time, but by now i realized it's worth more if you really think yourself into one and focus on it, I want to think as much as possible about the idea to prevent plot/logic holes, and to make it as lively and convincing as possible.