You create as many as is appropriate for the story. If you are picking the race of characters for any reason other than the story itself, you are sacrificing the quality of the story.

There are some practical concerns if your story is turning into a film. If you are filming in China you are probably going to have a lot of Chinese, if you are filming in Kenya, you are probably going to have a lot of Kenyans, if you are filming in Germany, you are probably going to have a lot of Germans, etc, etc.