Limiting registrations in Smartsheet Forms

Conchimc

Hi, I have built a Form in smartsheets for an event.

One of the fields (parallel sessions) has a dropdown list where participants can select the session they would like to attend.

The problem is that I would like to limit the number of people that can select the options from the dropdown, so that whenever 25 people have selected one of the options, this one is no longer available for other people to select.

Thank you