HI,

MAT score will remain valid for 1 year from the date of result declaration. So, ideally, if you appear for MAT SEP session, you should be able to apply for the academic batch of 2020-22. However, you need to check the application deadline of the institutes you choose to pursue your studies in. If the application date expires, then it is difficult for you to get admission.

The eligibility criteria for the exam are given below:

• Candidates must have a graduation degree in any discipline from a recognised university or Institute.

• Final
  year graduation students can also apply for MAT. However, they have to
  submit documents as a proof of completion of graduation within the
  prescribed time.

  Recommended

• There is no age limit prescribed to appear for MAT.

If you are able to satisfy this criteria, then yes you can give MAT(September) 2019. for admission in 2020-2022.

GOOD LUCK