In this sense, "fit in" means "increase the number of" or "increase the amount of". For example, "Our standard appointment time is 30 minutes. But if we reduce it to 20 minutes, we could fit in more patients." This means we could increase the number of patients. Be aware that in other situations, "fit in" can also mean "to feel a sense of belonging within a group". For example, "Jennifer fit in at her new school because she met lots of other girls who liked soccer." We can assume Jennifer likes soccer and she felt a sense of belonging because there were others who also like soccer.

Hello! In this sentence, "fit in" means to include or accommodate more people. I hope you find this helpful.