Under normal circumstances, each member of the base represents one labor force. It takes five labor forces to add an alliance slot. For instance, Shadow's main base currently has eight people, and together with the two from the supermarket base, that's a total of ten labor forces. The supermarket base contributes two labor forces, and the main base contributes three, allowing them to spend these five labor forces to form an alliance. After the alliance, the main base is left with five labor forces.
At this point, the supermarket base could then form an alliance with Base A, which has five labor forces. Theoretically, as long as the other party is willing and able to contribute five labor forces, the supermarket base could form alliances with an unlimited number of bases.