Why OSI model must have 7 layers?

Dear all tech suports

I have some questions are not clear in my head. and I need some advice. if is there any links resource page to all those questions below would be fantastic.

Q1.Why does the OSI reference model have seven layers, not six or eight?

Q2.According to the claim of optical fibre technology, it is almost error-free. and how do I Estimate the number of incorrect bits in a transmission of 100Gb.