im not sure exactly how many you could put in but you could keep a group of guppies that are all the same sex together although it has been known that a group of male guppies will become frustrated at not having a female and could start harrassing each other.
From what Ive read on these forums you will need to keep at least 5 male guppys to stop any aggression between them.You have plenty of room for 5 or more with a 70l tank
You can get just a group of females. However, more than likely, they'll be pregnant when you bring them home. I've had my group of females for about a month now, and so far have had four sets of babies to be born.
Also, you could put about 10 or 11 guppies in your tank.
