20.
http://hyperacusisfocus.org/research/earplug-use-2/
"While there are over 2200 posts on hyperacusis setbacks in the patient forum on chat-hyperacusis.net, no academic papers could be found using a pubmed search."
The fact that there have been no published studies regarding what causes permanent and temporary T spikes, means there is no scientific reason behind doctor advice to only protect your ears against noises that are known to damage the inner ear. They are basing this advice on studies that talk about what can damage healthy ears, whereas what can hurt us hasn't been studied (and the overwhelming number of testimonies on this site imply that sounds that can hurt us are Way quieter than the sounds that can damage healthy ears).