Can someone please clarify what is the plural form of proof of concept? Is it proofs of concept, proof of concepts or proofs of concepts?
It's proofs of concept.
As the moderator's closure comment indicates, the Wiktionary:Talk page is a good reference; e.g. compare to "mothers-in-law" or "attorneys-at-law."
If you consider proof of concept a compound phrase, it would be proofs of concept. In The Associated Press Stylebook, it says for compound words that involve separate words or words linked by a hyphen, make the most significant word plural:
- Significant word first: adjutants general, aides-de-camp, attorneys general, courts-martial, daughters-in-law, passers-by, postmasters general, presidents-elect, secretaries general, sergeants major
- Significant word in the middle: assistant attorneys general, deputy chiefs of staff
- Significant word last: assistant attorneys, assistant corporation counsels, deputy sheriffs, lieutenant colonels, major generals