[PlasmaKrab]

AFAIK rocket artillery goes more or less with the actual factual loadout, and it's the rate of fire that is conventionally tweaked to suit the number of rounds. In other words, if your MRL has 40 rockets loaded (or 8 or 12 or 100), each of the two first weapon slots gets 20 ammo (or 4 or 6 or 50) and the ROF is similarly 20 (or 4 or 6 or 50). That's the convention of MRLs as far as I can tell.
Only exceptions are some rare makeshift contraptions (think Afghan trucks with UV helo rocket pods strapped on the back) and those MRLs with onboard reload (RM-70, Chinese Type-90...).

Kramax is right, there should be the same convention all over the OOBs for heavy SPHs, though I can't find which unit gets 4 shots in the latest Russian OOB. 2S5 gets 30 rounds, 2S4 has 20 and 2A7 only gets 8, but that's as low as it gets.