My ex has recently gotten married and is considering to adopt his wife's daughter. My ex and his wife are having issues and I was wondering if he adopts her child and they end up getting a divorce, will he have to pay her child support? Will it affect the amount of support I am currently getting?

Yes he would have to pay CS. After the adoption, the child would be legally his, for all purposes, including the obligation to support the child.

It would not affect the CS payable to you, unless he were to go back in your CS case and get the judge to reduce your CS. It's extremely unlikely that he would be successful at that.