Answer
This functionality was added in 22R1 — it isn't present in 21R2 at all, so an environment showing it while believed to be on 21R2 SU7 is very likely actually running 22R1 (the visual style described matches 22R1's UI redesign too) rather than the selector being some kind of hidden/custom feature. Check the exact version reported in Account Settings to confirm which release the environment is actually on — if it's genuinely 22R1 or later, the 'Number of Rows to Fetch' selector is simply standard functionality for that release, not something that needs to be separately enabled or recreated.