We utilize Wufoo for this, but would love to have this ability within HubSpot forms as we build a workflow from Wufoo to HubSpot via zapier anyway. We use a form for vendors who attend our events. Within, we ask them which booth they'd like. Wufoo does a great job of this, as only one of each booth can be "ordered". When the booth form is submitted, that "booth" or property states "0 remaining".
I can see this being very beneficial in HubSpot for people registering for events (reserved seating), signing up for an e-course (limited number of attendees), or even when signing up to get a limited time document (10 downloads available!)
From Wufoo - About Quantity Fields: This limits the total number of times your users can choose a specific option. Wufoo will check to make sure that the user's selection hasn't hit the submission limit. If the limit is reached, a message will be displayed to the user informing them that the maximum number of submissions have been accepted. A empty value means there is no limit.
