How to track UASF support?
The total node count is not relevant. It's easy to spin up  nodes, or even fake nodes that just report a certain user agent string. That doesn't mean that they care about the rules, plan to keep following them if the hashrate majority goes another way, whether they'd even notice if they're forked off, or even whether they actually implement the rules.

What does matter is how much economic activity is following the fork's rules, and whether they're planning to keep doing so regardless of what happens.

Measuring that is something you cannot do inside the protocol. You need to look at company statements, press, ...