I would like to add a "if" function to my form.

I would like to add a "if" function to my form. For example, if the person completing RSVP is "Jack Smith" and I only have 2 RSVPs for the family of Jane Smith, I want the response to the question "How many total attending event" to have a drop down with options limited to 1 or 2. Similarly, if "Jane Doe" has maximum 4 RSVPs, then I want the total attending response to be limited to 1,2,3 or 4 only.

Hi! 

I'm afraid it is not possible to set field's limits conditionally, but you may create multiple dropdown fields with different options. Then, it would be possible to show the needed dropdown based on user's answer. 

Please take a look through the next help articles:

How-to-Show-or-Hide-Fields-Base-on-User-s-Answer

Conditional-Logic-for-Online-Forms