Availability for Advanced Forms Custom Field

Hi,

I was wondering if there is any possibility of adding specific availability to an advanced forms custom field. I am trying to setup a visit for a museum which has a maximum capacity of 25 people at anytime. Of those 25 maximum 20 can sign up for a guided tour. I was planning to create an activity with 25 as availability and then have a select field in which if the guided tour option is selected, the field would only be displayed until the 20 available spaces are filled. Is this possible with Booking Activities plugin? Is there a better approach to achieve the same result?
I would really appreaciate your help with this issue.
Thank you in advance.

Best Regards,

Paulo