To recruit surviviors you can either use the menu at the accomodation building next to your radio station,
or you recruit them via the menu on the left.
To choose the quantity of your new units you can use the slider or the input field. Now simply click on “Acquire Survivors" to start the process.
The little digit on the right of the survivor button will show you the current number of units which are available. 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 accomodation building.