Think of 'roomful' as a non-specific measurement of quantity, similar to 'handful' or 'truckload'. The charity received a roomful of toys during the holiday drive.
Pro Tip 2/3
Not Exact
'Roomful' suggests a full space but doesn't indicate an exact number or quantity. The organizers expected a roomful of resources by the weekend.
Pro Tip 3/3
Emphasizes Abundance
Using 'roomful' emphasizes abundance or plenty of something, which can be either positive or negative. The party left a roomful of garbage to clean up.