I don't have a proper answer to that one. Honestly I'd probably go with max up to number of BMs. As
FAQ has following
5.4.1 Void Shields
Q: We did have some confusion over
the interaction of Void Shields/Power
Fields and Blast Markers. Does having
Blast markers prevent having Void
Shields?
A: No. The idea is that downed Void
Shields can be repaired instead of
removing Blast Markers when you regroup.
So if you regroup you could use each pip of
the dice roll to either repair one shield or
remove one Blast Marker.
However since you don't get BMs from knocking down shields, this situation should be quite rare. As it would require to 6 formations to fire and only manage to score 4 hits.