The Male Rule Does Not Scale
It feels logical that doubling the water should soften a territorial fish, but a male betta's territory is not measured in gallons; it is wherever he can see. Put two males in a 20-gallon and each patrols until their patrols meet, and from that meeting the tank has a war with a front line that moves daily. One ends up shredded in a corner. So the male count in a 20-gallon is one, exactly as in a 10-gallon. What the extra ten gallons buys is not a second betta; it is everything else the tank can now hold around the first one.
Option One: A Community Built Around One Male
Twenty gallons is the size where a betta community becomes genuinely roomy. A school of small, peaceful fish that stay in the middle water and are too plain-finned to be mistaken for rivals, such as harlequin rasboras or ember tetras, can share the tank without ever registering as competition. A group of corydoras owns the bottom. The order of introduction matters: stock the community first and add the betta last, so he arrives to an established tank instead of watching intruders enter his. Even then, individual bettas vary, and a fish that spends its days hunting the school should be moved to a tank of its own; temperament, not the chart, has the final say.
Option Two: The Female Sorority, Honestly Assessed
The one route to several bettas in this tank is a sorority: a group of female bettas, at least five and better seven, added at the same time so no single fish owns the tank first. Females are territorial too, and the group's safety comes from numbers spreading the aggression so no one fish is always the target. It works best with females raised together, a heavily planted tank, and a keeper who watches daily and keeps a spare tank ready for a fish that turns relentless. Sororities fail often enough, through stress, disease moving through a stressed group, or one female's campaign against another, that honest advisors describe them as an advanced project with a good chance of needing intervention, not as the easy way to keep many bettas.
Planting Is What Makes Either Option Work
In a 20-gallon, plants are security infrastructure. Tall stems reaching the surface, floating plants dimming the light, and caves or wood at the bottom break the tank into zones a chased fish can disappear into, and give every fish a resting place of its own. For a community, planting gives the school somewhere to be that is not in the betta's face. For a sorority, dense planting is non-negotiable from day one, added before the fish rather than promised after. Aim for a tank where a fish at the front cannot see a fish at the back; that one test predicts more success than any stocking list.
Knowing When to Split Them Up
Whichever route you choose, the exit plan is part of the stocking decision. Torn fins that recur on the same fish, one fish that only eats when the others are distracted, or a betta spending the day in display against a tankmate are all signals to separate, permanently, rather than to wait for improvement. Keep a cycled spare tank or a willing shop in mind before you need one. Twenty gallons used well, one male in a calm community or a carefully watched sorority, is a beautiful tank. Whichever of those two tanks you are leaning towards, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.