In India, in which institutions are DM rheumatology seats available? Which kind of preparation is needed: the rheumatology subject specific or all 19 subjects like All India PG?

To the best of my knowledge the following are DM and DNB Rheumatology available in India (remember DM Clinical Immunology is same as DM Rheumatology, there is still no consensus on naming it immunology, rheumatology or both. You will get DM Rheumatology seats with a mix of this titles in India).

JIPMER 2 seats, one every 6 months
Separate entrance
(Rheumatology and Immunology based Mcqs – Kelley ‘ textbook on rheumatology, Harrison rheumatology and immunology section. Abbas textbook on immunology)

PGI ? 3 seats – 2 sponsored, 1 open
Separate entrance 
Based on medicine plus rheumatology (kelley’s textbook and Harrison)
All other NEET based (common medicine based exam)

Tamil nadu, MMC 3 seats Restricted to domiciles of TN ?

SGPGI – 4

IPGMER Calcutta -2,

NIIMS -3 (domiciles ?)

CMC Vellore 1-2 (1 might be sponsored)

DNB seats1) Hinduja Mumbai -1,
2) Gangaram delhi-1
3) Apollo delhi 1
4) Medanta 1-2 ?
5) Army hopsita delhi -1 (usually reserved)

As you can see mostly all seats will be filled by NEET which as we know ill be medicine based with common exam for all entrances.

If you want to specifically prepare for DM seats of JIPMER and PGI, you can subscribe to www.mymedtor.com, which has mcqs on rheumatology and immunology commonly asked in JIPMER and PGI exams.