I'm not even sure how Expert Grails go since I haven't done any in like six months, but it used to vary WILDLY from event to event. Sometimes it was guaranteed 5, sometimes it was 1-5, sometimes 3-5.
I farmed roughly 200-300 in a row this UE, was getting 5 every run since I usually solo them. Did however, accidently sent for support once and had someone take MVP from me in a matter of seconds... that run I only got 3.