Many of our guests will be traveling in from out of town and we want to be able to book the transportation from the hotel to the reception site early. How early can we request that they have their hotel rooms booked without inconveniencing everyone massively?
Re: How early to ask Out of town guests to book?
Are you just needing a headcount for a hotel shuttle, or are you trying to decide which hotels the shuttle will go to, or what? In either case, MyNameIsNot's suggestion of having people RSVP specifically for the shuttle is a good idea.
However, be aware that if you have a shuttle picking people from Hotel ABC to go to the venue, and only 15 people have RSVP'd for the shuttle, but 25 people are staying at ABC hotel, you're not practically going to be able to prevent people from joining on the shuttle on the day of. The driver isn't going to check for ID, or names on a list, etc.
I have generally seen people recommend one hotel, then aim high and provide enough transport seats to be sufficient if ALL the out of town guests stayed at that hotel. That way all your bases are covered.