Key Signatures are confusing!

https://preview.redd.it/p1pfasazeyog1.png?width=781&format=png&auto=webp&s=f296aaf77db272ef754b6a3b7b1c483eecd1a6bc

So I came across this method for determining sharps/flats in a key signature.

1. Why do we have 15 key signatures for 12 notes? Why are Cb and B repeated and have different ways of writing, which one to choose?

Is there a more efficient way of remembering key signatures? I know every scale and can sing them intuitively without looking at the keyboard.