Of course if they dont need to be together then its best to separate as soon as you can but I know housing 20 some individually is a pain.
As was said, once they start showing aggression, hogging food etc. They should be separated by size also.
The reason its best to house them individually from the start, IMO, is because they can get into fights along with one of them snagging a food item and another one wanting it. Ive seen it happen where on will shoot an item right out of the mouth of another. People have actually posted on here of animals doing it and thinking its cute but its not. This can pose serious risk to an animals tongue if its bitten while in the process.