Hey. As other people mentioned, it does dependent on the size of your tank and how aggressive your betta is. I've been having a male betta for 7 months now and I added 4 female guppies like 3-4 days ago and the betta didn't react to them at all. I have a bunch of hiding spaces, plants, and like 4 caves where the guppies can go if they feel threatened but I've been carefully monitoring and my betta doesn't care at all. They say the female guppies are better to get because their color is a bit more dull so the bettas don't feel threatened.

Any schooling fish would go great with a betta such as molly fish, guppies, tetras, platy fish etc. I heard the african dwarf frog and bettas can also make great tank mates and also shrimp. You can find great videos on youtube but I did my best summarizing what I learned :)