Need multiple people to complete a form but not in order

See form here please.  I know how to enter text fields.  What I don't know is how to send this out to 100 people via email, and then let them complete it on a first come, first serve basis, until either a time deadline passes, or we reach 100 units (which I would be monitoring), whichever comes first.  For example, I send it to 100 people...

1. John is really on top of his Inbox, opens this right away and he immediately sees it for the opportunity that it is.  He fills in a row with his info and signs on for 10 units.
2. Bill opens this email 2 hours later, and sees that John already signed on for 10 units.  He signs on for 5.
3. Over the next 6 hours, 20 others open the email, and 10 more sign on for varying amounts of units.
4. Althewhile I am monitoring the document and see that the total unit count is at 90, almost to the limit of 100
5. The next day Greg opens up the email and this document, sees that 12 others have already signed on, and that there are only 10 units available, and he goes ahead and signs on for those remaining 10.
6. So only 13 rows have been completed, but the offering is now over as all 100 units have been subscribed to.
7. Seeing this, I take the document offline at this point.  Any others who are behind on their Inbox'es and open my email and link to this document up after this point will be taken to a "this document is no longer available"-type message screen

Any assistance would be appreciated,