In Test cricket, how many batsmen have been dismissed for a duck on the first ball of the opening day?

More than you might think and dating back to the 1894/95 season in Australia.

According to Wikipedia's page titled "Duck (Cricket)" being dismissed off the first ball of an innings is known as either a "Royal Duck" or a "Platinum Duck."
https://en.wikipedia.org/wiki/Duck_(cricket)

ESPN Cricinfo has statistics for nearly everything you'd ever want to know about the game of Cricket.

Their list of batsmen "Dismissed by the first ball of a match in Tests" identifies some 35 unfortunate souls (or successful bowlers). The most recent such dismissal was in the 2024/25 season, again in Australia. We can expect this number to grow as the years progress.
https://www.espncricinfo.com/records/dismissed-by-the-first-ball-of-a-match-283164

So far 35 batsmen have been dismissed on the first ball of a Test match. The 35th was Yashavi Jaiswai who was dismissed lbw by Mitchell Starc at Adelaide Oval on 6th December 2024, Australia v India.

https://www.espncricinfo.com/records/dismissed-by-the-first-ball-of-a-match-283164