The signup fails if the desired number of volunteers have already signed up for the Job/Shift, and the volunteer status is confirmed.
The volunteers can see the desired and signed-up volunteer counts on the job/shift tile on the V4S Kiosk application. And they should not signup for these shifts as the desired number of volunteers have already signed up.
If the volunteer tries to signup for a Shift using the "First-time Volunteer? Click here!" string even after the desired number of volunteers is full, then only the volunteer contact is added in Salesforce and the volunteer is not signed up for the Job/shift.
Also, there is no message which says the desired number of volunteers are already signed up for the Job/Shift, when the volunteer uses the "First-time volunteer/ click here!" string to add and signup themselves for Job/Shift on the V4S Kiosk application.