I got 323 rank in IIT JAM in Mathematics. Which IIT can I get?

You have a good rank you can get one of the best IITs with this. For the relevant information, you can visit this link: https://www.shiksha.com/science/articles/colleges-accepting-iit-jam-scores-ranks-for-admission-blogId-13971

You can get admission in IIT Guwahati, IIT Gandhinagar and others. It is tough to get admission in IIT Delhi, IIT Roorkee and IIT Mumbai. You can get admission in other IITs easily.

You may get a seat for M.Sc (Mathematics) at IIT Indore, IIT-Ropar, IIT-Patna, IIT-Bhuwneshwar, IIT-Gandhinagar and IIT-Hyderabad. You may choose according to your home location but I would suggest you prefer IIT-Hyderabad as it is one the best IITs (the new IITs). Congratulations on such a good ran