I don't think Wwise offers a way to do exactly what you ask. But this situation is usually fixed by proper priority handling. The voices which will be kicked will be the lowest priority voices. When a blend container is triggering more than one sound, both sounds are attached to the same event and to the same game object, which also means they have the exact same priority... unless you override it lower in the hierarchy under the blend container.
From there, if these are 3D sounds and you activate the priority to vary over the distance, both sounds will be on the same rank in the line to be kicked out, which reduce drastically the occurences of partial content of blend container playing as when one is being kicked, the other one is necessarily the next in line to be kicked.
But if your goal was really to count many voices as only one in the limiter, no it is not possible with this system. The system is a voice limiting system, so as it says, the limit is counted in number of voices, not events or something else.
Hope that helps.