if i apply for a X1 visa, will i be able to get multiple entries till my course for like 18months?

Santhosh, you just need to change your X1 type to a valid resident permit within 30 days when you arrive. The officers of the local entry and exit management bureau usually give enough duration for you to complete your study. If you acqure a short resident permit, you can extend at local entry and exit management bureau too.