A functional jewel count has a floor and a sensible ceiling. Seven jewels cover the escapement; fifteen add the train; seventeen add the centre wheel; twenty-one typically cap the train and add automatic or chronograph works. Each step up to about twenty-one addresses a real pivot that benefits from a jewel.
Counts above that need scrutiny. Some are legitimate — jewelled chronograph mechanisms, jewelled winding works. But the mid-twentieth century saw watches advertised with implausibly high counts achieved by jewelling points that did no work, because 'more jewels' had become a selling point detached from function. A 40-jewel simple watch is a marketing artefact, not a superior machine.
Placement matters more than the number. Where the jewels are — whether the centre wheel is jewelled, whether the automatic works are jewelled — tells you about the maker's priorities; the total alone does not. Two 21-jewel movements can differ greatly in grade.
Jewel setting style is also evidence. Screwed gold chatons signal high-grade nineteenth and early twentieth-century work; friction-set jewels are the twentieth-century industrial norm. Neither is 'better' in service, but the setting style helps date and grade the movement.
For cataloguing: record the count, the placement if visible, and the setting style, and resist treating the number as a quality score. Grade is the sum of finishing, adjustment, materials and construction — jewels are one line in that account, not the total.