Lightroom 6 does not natively offer a smart collection rule for “more than X keywords” or “more than X tagged people.” Based on the community answer, this type of search is possible by using a plugin such as Any Filter by John R. Ellis, which adds more advanced filtering options than Lightroom’s built-in smart collections.

So if you need to find photos with, say, more than 3 face tags to identify group shots, the practical solution is to use that plugin rather than Lightroom’s default search tools.