I carry two cartridges. They are attached to a common holder along with my forward bottle cage...



If need be, I could carry two more by buying similar mounts for my seat-tube bottle cage mount (might rub my legs though) or I could use a generic one like this...



And if that's not enough, I can always install some cage mounts onto the saddle...



... and in conjunction with two of the above cartridge mounts, carry an additional four cartridges. Seems over the top though.