I've toured with as much as 20 litres. I have the three standard cage mounts on the bike which hold 1 litre bottles. I used radiator hose clamps to clamp 2 more cages on pointing sideways on the frame up near the steering part of the frame.

I have another 2 clamped on a bracket behind the seat, and modified my water back pack to hold two 2 litre bladders. The rear of my BOB trailer has two oversized cage mounts which each hold a 1.5 litre bottle.

The rest gets carried in 5 litre containers in the BOB trailer. The reason for so much water is that it's hot and a long way between water stops. Carrying this much water, I have come close to running out once.