I would suggest you read up on cycling because it is SO important to understand water chemistry to keep a sorority successfully (IMO)
Most say 4-7 girls in a ten gallon, I'd say 5-6 ideally, so 8-14 in a 20 gallon, but 10-12 would be better. More girls = less aggression but you don't want too high of a bioload. A sorority needs AT LEAST 10 gallons and 4 girls. Most people would prefer larger than a 10, though. At least I would.
Cycling is basically this - you have beneficial bacteria (BB) that will battle the ammonia the fish produce. The BB colonize faster in temps that are higher (82-84) so if you do a fishless cycle, raise the temp. The BB grow on surface area (decor, substrate, tank walls, etc...) but MOST of the BB grow in the filter media so you need a filter. They need oxygen (from the filter movement) and cannot colonize in pH less than 6. DO NOT change all of the filter media as this will ruin your cycle and remove most of your BB and cause mini spikes. It is best to put sponges (IMO) in the filter instead of carbon cartridges as carbon will remove meds if you ever need to treat the tank for anything. If you ever move the tank, leave the filter media in water and do not let it dry out and NEVER rinse the media in straight tap water. The BB will be killed by straight tap water and you should only rinse the filter media in conditioned tap water. If you ever need to change the filter media, it's best to let the new media sit in the filter for a month with the old media so that your BB will transfer over. It is a slow growing bacteria.
Ammonia feeds the BB and once they colonize, they will convert ammonia to nitrItes. Then, more BB will colonize that change the nitrItes to nitrAtes. NitrAtes are harmful in amounts over 40 ppm (parts per million) but ammonia and nitrItes will be harmful in ANY amount. Cycling will ALWAYS cause ammonia/nitrite spikes that can easily go undetected. While cycling, you need to check the water parameters daily, ESPECIALLY if you have fish in there, because a water change is needed any time you see ammonia or nitrites (if there are fish).
To do a fishless cycle, you need a source of ammonia since there are no fish. You just need to dose the tank to 4ppm ammonia and basically wait. It can take 1-2 months or longer. You will first see a nitrite spike, then you will see the ammonia start to fall, and then the nitrites will fall and nitrates will show up. Once you see 0 ammonia, 0 nitrites, and 0-40 nitrates, the tank is cycled. If you can dose the tank at 4ppm ammonia and it falls to 0 in 24 hours with no nitrites, the tank is cycled.
Cycling takes a while, but it is for the safety of the fish and not the ease of the owner. You don't need as many water changes in a cycled tank, but it is really for the safety of the fish so they aren't ever exposed to ammonia and nitrites.
Live plants will also help with ammonia. I am cycling my sorority tank and adding live plants for that exact reason - to keep my fish safe, not to be lazy and do less water changes.
lol, hope that helps.