Black Friday is coming soon so I wonder if there will be a discount or any promotion for CSC purchase? Like if you purchase CSC during that will get you more CSC then purchasing in regular time?

We can't really say for sure until it's announced, but generally based on past years, that's not part of the sale. A lot of stuff in the Cash Shop is discounted and older things are brought back but there isn't usually additional CSC given out on top of that

Thank you for the info!:)