Another strategy to ensure fresh developer is to accumulate exposed films and delay mixing the developer until you have enough exposed films to consume the developing capacity of the kit within a few days.

Kodak shows D76 having a capacity of 4 rolls per quart (or liter) of stock solution, or 16 rolls per gallon (or 4 liters).

Kodak’s capacity notes for D76 at various concentrations is shown on page 7 here

http://www.kodak.com/global/en/profe...bs/j78/j78.pdf