Who was the last Indian top order batsman to play only one Test?
This question was prompted by something someone said to be about selection. I decided to find a good answer to this.

Here’s the full answer [google spreadsheet]. The modal batting position is the batting position most frequently occupied by the batsman. The other columns are self-explanatory.
Gursharan Singh was the last Indian batsmen to play only 1 Test. He played in Auckland against New Zealand in February 1990. He batted at number 7 and made 18. That was the Test in which Ian Smith made 173 in 136 balls in New Zealand’s 1st innings coming in at 7/131, and Azharuddin made 192.
There are only two players who have played 2 Tests as batsmen since then. Hrishikesh Kanitkar played 2 Tests in Australia in 1999 at number 6, and S. Badrinath played 2 Tests against South Africa in India in 2010. The next shortest Test batting career belongs to Hemang Badani who played 4 Tests for India in Zimbabwe and Sri Lanka in 2001. Naman Ojha kept wickets for India in the third Test of the 2015 tour of Sri Lanka as a stand-in for the injured Wriddhiman Saha.
Before 1990, solitary Test caps for batsmen were less uncommon, though still rare. All in all 10 batsmen made their Test debuts in India’s top order and played only 1 Test match. These are
LP Jai in December 1933
MR Rege in January 1949
AK Sengupta and Arvind Apte in July 1959
Ramesh Saxena in June 1967
Kenia Jayantilal in February 1971
T Srinivasan in March 1981
Ghulam Parkar in June 1982
Ajay Sharma in January 1988
Gursharan Singh in February 1990
Sadly, even today, the public perception about selection is that it is a largely a capricious process rather than a job which involves significant professional expertise.