I got  a requirement to book the corporate cards in SharePoint, basically we have total 10 corporate cards, so when someone submits the form the number goes down from 10 to 9 and so on, when all cards gone. we need send email to requester states all cards are booked this time. For this requirement, I created 2 lists in SharePoint, 1st list for the how many cards are remaining calculation and 2nd list is actual sign up list, I have added a lookup column in the sign up list from the 1st list, once the card booked in the 1st list, I have added the filter in the 2nd list to show only available cards not the reserved one, My issue is when all the 10 cards booked, my lookup drop down shows no value. 

My issues are:

1. When all cards booked in the 1st list, if someone wants to book a card then how can I notify them that all cards are booked this time, because my lookup field in not showing any value in drop down so I am not able to do any query. Is there any way that I can calculate a number of remaining cards in a 1st list when someone tries to open new form and redirect them to a page states cards are not available this time? or any other methods.

 Note I am using Nintex forms and Nintex Workflows for office 365.

Any help is much appreciated.