That form can be totally confusing! Let's see...
So you have 17 groups that need rooms and you need to hit 50 room nights? (Just to clarify, putting a certain number of room nights into your form won't guarantee you the free night - they have to actually be reserved by your guests.
)
If you have absolutely no idea where anyone would want to stay or for how long, you'll have to do a guesstimate just to cover your requirements. Then you can change your room block later on if necessary (like if everyone ends up picking just one hotel).
For your guesstimate, what you could do is split the 17 groups roughly by the three resorts, and set aside, say, 5 rooms per night at one hotel and 6 rooms per night at each of the other two. Then fill in as many nights within the period as it takes to get to 50. In your case, 17 groups would only need to book 3 nights apiece.
Don't forget that your own stay counts, so you can stick that in. Also, instead of just concentrating all 17 groups in each of the 3 nights around the wedding, you could set aside a few rooms in the days on either side of the wedding for people who want to stay longer, and then reduce the number of rooms set aside in those 3 days close to the wedding.
I hope this makes sense - it's hard without the visuals!
