A mixture of things can be used. It's much easier for films with a slower rollout of reviews, archive.org can get you about as close as you'd like, for example it was very easy to find where Ad Astra was at 34 reviews.
If you are really miserable or want any kind of excuse to not get any work done, it can be done manually by checking All Critics and heading straight to the back, where all the first reviews are stored. Faaairly alright for lower numbers like 34, but not recommended for high numbers lol.