First, if you are using radio buttons, the user will not be able to choose more than one option. That is the point of a radio button. Checkboxes are when the user can pick multiple options. You said "radio box" so I'm not sure which one you are using. For your example, you should NOT use checkboxes... use radio buttons.
The selection limits are for limiting how many times each option is able to be chosen for that event out of all registrants. So if your values are:
Regular Member|Student
The selection limits would be:
0|0
You have to have a limit given for each option. Use 0 for unlimited, which is what I assume you will want to do. If you put in "1" for a limit, once a user chooses it, that option would not show anymore for other users.
Hope you understand now