Unlimited users means you can create any number of users from your GoSurvey account. Be it for any role - like Survey Manager, Data collector, Data viewer etc.
Number of Device License
If you want to use GoSurvey on multiple device, then you will have to purchase the required number of device licenses. If you want to collect data on 6 devices, then you will have to purchase 6 device license.
You purchased a plan of GoSurvey, for 6 device license. In this case, you can use GoSurvey on 6 different devices. Though you can create any number of users from your account dashboard, but at a time only 6 device can remain active. If you wish to collect data or feedback from different device(s), you will have to de-activate any of the presently active device(s) and then you can activate any other device(s) of your choice.