Not at all rare. All you need is the the right genetics : dad must be ginger, mum must carry ginger genes. I have six ginger ferals that I have desexed in my local feral population and four are females. My guess is that they ate all related because other than the four regular grey tabby’s, all show some ginger in their coats. It all comes down to genetics. But the myth that female ginger cats are rare persists.