In reality: None, was always condition to take up arms. Every depictions that shows no helmet is either stylized for fashion or the helmet is well hidden beneath a hat.
https://en.wikipedia.org/wiki/Secrete_%28helmet%29
This goes only for that period though. In antiquitity and the dark age you could get away with being bare-headed.
Fun-fact: (Selbst im 18.Jhd trugen Kürassiere in Wahrheit Gitter/casquets über ihren Dreispitzen, um die zu verstärken, weil die sonst alle durch Kopfschläge gestorben wären. https://upload.wikimedia.org/wikipedia/commons/a/aa/Cuirassier_hat_and_casquet.png?20200901204113 )
In fantasy: Plenty.
Deturnation
2023-02-14 22:51:17 +0000 UTC
He's missing a helmet. How likely is it that regular troops appear on the battlefield without helmets?