By checking the WordPress stats we get the idea that how many times a WordPress plugin is downloaded. But this is not the number of how many users for that plugin, right. Same user will download the plugin when a new version releases.
So do we have any tools or stats to get the total number of unique users for a WordPress plugin??
I did a research on the matter. And the answer is no.
For your own plugin, you can use tracking as @PeterVanDerDoes points out.
Curiously, the plugin I used as example in the research, WordPress SEO by Yoast, is the same that does this kind of tracking. And here’s a nice discussion about it.
I’ll reproduce the relevant part of the plugin development official guidelines:
The only way I can think of that you could track something like this is by having the plugin phone-home with some stats to your own server.
Just make sure users can select to opt-out of tracking.