I am going to write sat for the next fall(2019). Will the marks be valid for the 2020 fall?

Yes of course. SAT scores do not have a "validity", but there is a catch.

Some universities demand SAT scores not more than say X years old. So in those particular cases, you'll have to have a valid SAT score obtained within that period. But rest assured, that period is always more than 2 years. So in your case, your score will be valid.