You could fit 2, maybe even 3 leopard geckos, although I wouldn't recommend it. Leopard geckos are healthiest and happiest living by themselves. Housing multiple geckos together, while it certainly can be done, opens up a door of new potential problems such as cage mate aggression, dominance, competition over food, and easier transmitted illnesses. All those variables you must keep in mind if you want to house multiple geckos in the same enclosure.
i would recommend keeping the geckos for 30 day(minimum) quarantine before putting them together.. after that IF the geckos get along AND you pay attention an make sure all are eating have 1 male 2 females or 3 females in a 20 long.
A general rule of thumb i heard is 10 gallons per gecko. And like stated they should be quarantined before hand. And also stated that they are perfectly happy living by themselves better than with other geckos.
I dont like using volume to measure leo space. 10gals can come in many forms, as can 20 and so on. Just as stated previously 20gals can be 15" long or 30" long. When talking leos i belive surface area would be a tad more important. Not trying to be rude or anything but i thought i may mention it.