It's both R and the number of cases
If either of those is getting high, then additional measures might be needed.
But even wi a high R, if there are only a handful of cases, then further measures will not be needed, or at least not yet.
I wouid hope that there is careful monitoring everywhere, by now, so that trends are properly examined, and measures brought in when appropriate.
And yes, if the rise becomes sustained (numbers well up) or more generalised (cropping up in oodles of places even if low numbers in each), then that wouid be concerning. And measures might follow.
For resort areas, it might mean a ban on visitors to hotspot areas.
That really would put staycation back to its original meaning