Some females might be aggressive, but most can live peacefully with others. I have a 10 gallon community tank and it used to have 4 female bettas, two cories, and a baby pleco. Now it only has 2 cories and 2 females, because of velvet and I had to start all over.
The general rule is 1 inch of fish per 1 gallon of water. But being these are female bettas, it would be 1 inch/1.5 gallons. So in a 5 gallon I would put 3 MAYBE 4. But considering you have cories...I would recommend 3. Anyway, make sure you don't add them all at once because it might cause an ammonia inbalance. Quarentining will help pace you. Get a female, quarentine it for 2 weeks, release it in the tank. Get a female, quarentine it, release it. This way you are not overloading the biological nitrate cycle.
Good luck!
Alexa