I work in marketing. For general audience it might be something like 3% of users have a tracking blocker, and specifically for developers it's something like 30%.

It actually becomes a bit of an issue when some idiot somewhere designs a business process around client side tracking. I'm currently fixing a broken process with a client where their users can request an upgrade, and ~1/3 of them (the technical users from above) never even process because some yuckster built it all inside of the tracker.