You could use a tank of 45 litres to hold them as an isolation tank for a month or so before moving them to a larger tank. I must agree that the 45 litres is a bit small for a platy. I would be comfortable with about 2 males and 6 females in a 60 litre tank though. Part of the issue for an active fish like a platy is swimming room for a decent quality of life for the fish.