The answer will be found in your desired batch size.
A 15L/~4 gallon kettle will accommodate around 11-12L/~3 gallons at the start of the boil (you could bump that up a bit using anti-foam agents, fwiw). If you allow somewhere around 2-3 liters boil-off, you're down to 9-10 liters. Figure another liter or two loss between kettle and packaging and you're down to 7-9L.
BUT...you can do things like no-sparge mashing with enough grain that at the end of the boil the specific gravity is high enough to let you add "top-up" water and end up with a 19L batch - enough to fill a keg...
Cheers!