How many devices can I use at the same time?

The device limit is 3: an account can be signed in on up to 3 devices, and can play on 1 of them at a time.

If you start watching on another device, the app offers "Watch here instead", which stops the stream on the first one.

To sign a device out and free its place, open Settings > Devices (/account/settings/devices) or Account > Devices in the app.