I'm US citizen. If I want to stay in China for 90 days with L visa, how many entries should I apply?

Winnie, the stay duration has not relation with the number of entry. You can apply for a single entry L visa and asks for 90 days stay duration. If you apply for a multiple entry visa, the stay duration per entry is 30 days, you have to exit mainland China when the 30 days stay duration will expire, then reenter Mainland Chian for another 30 days stay duration, then exit again and re-enter for another 30 days stay duration.