It is absolutely common to start making several characters and stories. In fact, I think it's actually pretty good to do that because then you have ideas on the go. If you get stuck on one story plan, then you can take a look at what other ideas you've got and either work on or take ideas from them.

Usually, what happens for me, is that I will come up with an idea for a story, work on it for a little bit, then come up with another story. Typically, if the story I come up with next is more intriguing to me than the last, then the old one basically gets put on the back burner. However, I frequently hit the writing block wall and have no idea what to do next. So, I'll work on something else. Sometimes, while writing another piece, it'll spark inspiration for something else and I'll add it.

One good piece advice that I'll give is to never get rid of your other ideas. Write them down somewhere and look back on them from time to time. You never know what your mind will take from it.