Hey,

I don't think any IISER in India provides specialised MS programs. They usually offer integrated BS-Ms or integrated PhD programs.

Also there is no astrophysics specialization as such, you do get to study subjects of astronomy and astrophysics. Admissions are done under Integrated PhD in Physics or physical sciences.

Total 244 seats are available at IISER Mohali and these are distributed among their BS-MS, integrated PhD and PhD programs. There are no fixed number seats for any program.

For other IISERs like IISER Pune it's 288, Bhopal 252, Kolkata 240.

For further information you can look upto the official website, admission dates and process is mentioned over there in detail.

Hope this helps.