It's not very comforting to have 40-70 rounds of ammo if you only have 5-7 in the ready rack in the turret, and need to pop open the floor panels under the crew to get at the remaining rounds. Figure on 2-3 HE, 2-3 AP, and 1 smoke at the ready, so you take 2 or 3 shots at a target with the appropriate ammo type, and then you're forced to break off from combat while your hull crew stands on the seats and hands ammo up to the turret crewmen, or else use ammo that's not suited for the task.
As pointed out, a 'Mech has all of its ammo, any carried alternate ammo types, available at the touch of a button.
As also pointed out, at least a dozen factors influence the "ideal" amount of ammo to carry. It's not as simple as "1 shot per expected turn", especially if the battle can be of indeterminate length.