Dear Aspirant,
You can definitely apply for improvement exam even in one subject only but in the succeeding year ie. 2020. Read the procedure below to apply for improvement exam of any board.

To give the HSC improvement exam 2020. Please  follow these steps :-

Fill the online private examination form on official website of your HSC board and pay the required fees.

Choose your subject in which you want to appear for improvement exam.

Once you fill and submit the form you are done.

Now Wait for theadmit cardthat comes in mid of February on the same website ie. Official Board website,Your roll no. , center of exam and time table of exam everything will be written on the admit card.
Hope this helps! All the best.