My two cents as a former mortar man:

It is useful to use three or four mortars simutaneously, because it is often intended to hit suprisingly at the same time with as much as impact as possible. With four mortars you get in an aerea of 80x80 meters enough splitters to hit with a high percentage any uncovered target with 16 rounds. With just one mortar that doesn't work. So I recommend using more then one mortar at one time. That doen't apply for platoon mortars.

Mortars are simple and effective weapons, but normally need specialist operators and and a lot of ammunition. I would normally organise them on battalion level. For light infantry I would even on battalion level go for 60mm oder 81mm. There are different possiblities for organizing them, but I would group them, that always at least 2 mortars are always together, either in a section or platoon. It can be useful to detach a mortar section or platoon to a rifle company. The heavier the mortar the less like it is detached to a company.