with cherries I recommend you start with a trio, or even six. 2 female to 1 male ratio. They are easy to sex. Females are larger, brighter, and have a curved underbelly.
If conditions are good you will have four females with bellies of eggs soon enough! Also, females will have a saddle, you can see the yellowish blob at the base of the head/back. That is unfertilized eggs.