this article says there are 24 cabinet members and implies they all could vote to remove.

Under the amendment's fourth stipulation, it would only take 14 people to depose the president — Vice President Mike Pence and 13 of Trump's 24 Cabinet members.

https://www.independent.co.uk/news/world/americas/donald-trump-impeachment-25th-amendment-remove-sitting-president-mike-pence-white-house-a7996681.html