There is no limit to the devices you can pair with your account
However, if there are more than 20 devices paired with an account it might cause the data to be synchronized slower than expected.
If you have an Airthings Hub
There is a limit of 10 devices that can be paired with 1 Hub.
Adding a monitor to your account
To add a monitor to your account, press "+" icon on the right top corner of your app and follow the in-app instructions.
In the Airthings app
In the Wave Gen 1 app