The air purifier has a CADR of 500 (at the Max level): According to ANSI/AHAM guidelines, this is sufficient for a room of 42.5 m². Calculated according to NRCC guidelines, it is sufficient for 130 m². Philips, of course, gives a somewhat higher value. However, I would recommend using one air purifier per room, as they can then be operated at a lower level (unless the rooms are well connected and less than ~70 m² in total).

We have one for the hole 5.5 room and we find air quality better, and allergies tamed. However, I am no expert, but if you suffer from allergies at night it may be worth getting 2 units.

It makes more sense to use a separate unit in each room.

Theoretically, yes, but practically, no. We use several units (so also a separate unit for the bedroom).