you can keep any ratio want - - they get along regardless how how many males or how many females.
To determine gender, it is easiest for newbies to figure out the males. Look at their armpit - -males will have a gland that resembles a pimple...can be either white, pink, gray or any shade in between.
Females have a small extension between their back legs on their rear end called a cloaca and is used when passing waste and eggs.