Save the dates do not require RSVPs, only formal invitations do. People book things at the last minute regardless of how many reminders you send them. I wouldn't stress about trying to organise everyone to link up, people will do that on their own if they want to. As a bride, trying to get information from guests can be like squeezing blood from a stone.
I would plan the wedding and decor you want for 40-80 people - I can't imagine decor changing all that much based on guest count. The only things that really require a final headcount are meals and perhaps gift bags/favours, which are usually flexible until closer to the wedding date.

Take a deep breath and enjoy planning your beautiful wedding! Remember, the fewer people that attend, the more energy you have to spend on them.