I didn't vote because it's hard to pick. Itdepends on the tank and the geckos. I have a 20L with only one male in it because he lost his tail and it is still growing back and he seems happier without the female that he came with in there. I have couple other 20L that have 2 females, we have a 29 that has a non-breeding pair ( we bought them together and they were 2 when we bought them and had always been together the breeder said but never bred) We have had them for a year and they still have never bred). We have a 55 that houses 3 ( soon to be 4) females. It just depends.
a 20 gal long is 30" long 12" high and 12" deep and paul is right about the geckos health comming first, but as a general space limit, if 3 females or 2 females and 1 male all get along they would have enough space in a 20 gal long as long as they each have a moist and a regular hide but you would also be limited on what else you could have in there for them to climb or lay on such as drift wood so if you want extra things in there tank then 2 to a 20 gal would be better.
It does depend on the cage size and the sex. In a 100 gallon you can keep like 4-6 females and one male. But if you didn't know don't keep 2 males in the same terrarium.
I believe if you have MANY MANY hides, basking areas, feeding bowls, moist hides, plenty of space and all your leos are happy with being together (no fighting) I think you can have many leos in a tank... Obviously you need a large tank in order to fit 5 or more leos in one habitat. But if you have 2 hides, 2 everything per leo I dont see there being a problem.. Thats what I have!! I don't have any issues..
I was going to put 2 in an almost 9 gallon tank, but the bigger of the two didn't like the company. So right now they're housed seperatly. Planning on getting them their own 10 gallon glass tank in the next few months.