How many devices can I use one license on?

Three at a time. Settings → Devices lists them and shows the count as N of 3 active.

A device is registered the first time you sign in on it, so the list fills itself in as you go. A different browser on the same computer counts separately — registration is per browser profile, not per person — which is the usual reason people reach three sooner than they expected.

Each row names the device by browser and operating system, with when it was added and when it was last seen, and the one you are on is badged THIS DEVICE.

The cap is enforced on the server, so there is no client-side trick that gets a fourth and no bug that can silently cost you one.

See the Devices tab.