Jobo Drum Capacities - that tells you how much you need to cover all the film in the tank. You also need to think about developer capacity which is about 16 rolls/L (64 sheets/L) of working solution with most professional-grade C41 chemistry (Kodak, Fuji, Rollei, etc). So with 12 sheets, you need at least (12/64)L=188mL and you will exhaust the chemistry in one go. Clearly 188mL isn't going to physically cover the film, so you use the tank-minimum instead.

If you're processing the developer to exhaustion by extending times (like the Fuji kit does), then you would make up a 750mL batch and use it 4 times to process a total of 48 sheets, which is 3/4 of 64 sheets just like 750mL is 3/4 of a L. If you're doing replenishment, then you make up a 1L batch and after each process, replace some fraction of it according to how much film you processes. 12 sheets is 3 rolls and at 50mL/roll replenishment, you would take 150mL of fresh developer (without starter) and top that up with 850mL of reused developer then use that 1L for the next batch.