Yes, it is.
There's two way to go about diong this.
1. Mod
- Look for "Advanced Setup" mod or something like that, it's up on Workshop, it should be pretty easy to understand
If you don't want to use Mods
2. Go into XML, look for the Civ5World.xml, make a backup of the file somewhere, go into editing it, and look for the largest size, the command you are lookin for is DefaultPlayers - set that to 22.
Just know that a lot of the stuff might be unbalanced, and you gotta be careful, as far as I know, it's a baaad idea to edit core files I personalyl thing as long as you back up and can validate anytime nothing bad should happen.