Which means yes, they *can* use all their eyestalks in the same round, but they have to be surrounded.
Just how surrounded they need to be depends on just how nitpicky you want to be defining 90deg arcs...
i.e., can you define two adjacent characters 30' in front of the beholder to be in different arcs (beholder draws the arcs in the most advantageous way) or are two adjacent characters by definition in the same 90deg arc (only three rays per any potentially possible 90deg arc)
IMO, facing related issues should be dumped entirely and it should be defined as max of 3 rays per target, or 2rays each for any adjacent targets.