The new Victrix Viking and Saxon sets have a lot of bare heads and heads in caps of various styles. The Frostgrave barbarians set also has lots of bare heads. Of course most of these (in both cases) tend to be pretty hairy and beardy / moustachioed. If you’re looking for clean shaven bare heads, there are not so many of those around, perhaps one or two in some of the Dark Age / Medieval plastic sets out there, like the Conquest Games archers.
I think I counted around almost 100 different separate heads across all the Perry WOTR and HYW sets. Of those, IIRC there are barely half a dozen in soft hats / caps / bareheaded... and the hats that are there are pretty definitively late period medieval in style, so probably won’t sit comfortably with Fireforge medievals, which are kind of 1200-ish in look