From that link:

What they didn't mention though: what happens if a bunch of miners doesn't upgrade? Because that's the issue; if a majority of economic nodes starts enforcing something and a minority of miners starts mining it, then there is a way for an attacker to trick non-migrated miners to violate the new rule (as the new rule is always narrower.) This would cause nasty chaintip forks, like what we saw with CSV when miners had signalled support but the block templates construction was done outside of consensus (and thus were illegally spending time-locked coin according to consensus rules.)

So, whatever you do, you have to get at least a majority of miners on-board and, preferably, not leave anyone behind, unless they choose to.

I read about 80% during segwit in 2017, but not sure it was a number of UASF signaling nodes or a requirement.