To recruit Nomads you can either click on the Nomad Housing building in your district or, you can click on the "Nomads" button on the left side of your screen.
To choose the quantity of your new units you can use the slider or the input field. Now simply click on “recruit” to start the process.
The little digit on the right of the nomad-button will show you the current number of units available to you. A red colored digit means that you haven’t reached your unit limit yet and can still recruit some more. To raise the limit, simply upgrade the Nomads building.