Not enough with enough 121 hours to upgrade who are WILLING to upgrade at the regional level when they can jump to a major. That may be a temporary issue, but for many regionals it still might be a terminal one. Temporary problems can become permanent ones. Try putting a plastic bag over your head - just for 10 minutes or so - and SS how that works out.*

*No, don’t really. Not even a pangolin’s metabolism could survive that, tough as the little buggers are. But you get the point. There are fixed costs to running an airline that don’t cease just because one part (albeit an essential one) is in short supply. Many regionals won’t have the ability to endure a significantly sustained interruption in that supply