>>49612034ancient lore suggested that Reimu was the 13th Shrine Maiden, but considering it's not been referenced ever again that might not be canon anymore. I think it was even suggested to be wrong when it was first said.
Anyway I subscribe to the theory that the average life expectancy for Hakurei Shrine Maidens was pretty damn low pre- Spell Card Rules, so I think there could potentially have been a good few between Mizuchi's era and the present.
I think Mizuchi's Shrine Maiden was an exception to the low life expectancy (Reimu is too, but for Spell Card Rules reasons) and there might have been four or five between the two.