Save the date information

That information would typically go on your wedding website. Then you'd include on the save the date a link to the website.

View Quoted Comment Thank you Veronica!
Also, how can I know how many people will need/want a hotel room ?

View Quoted Comment I'm not sure why you'd need to know that information. I would just recommend some different hotels in the area and let them book from there. If you are trying to do a hotel block you could reserve how many rooms and let people know on the website that x number of rooms have been reserved at x hotel. If you are going to do a hotel block I would probably do that at two hotels one that's more cost effective and one that's a bit nicer. But ultimately its up to your guests to handle their own accommodations.

View Quoted Comment My venue is giving me a voucher if I have more than 10 rooms. That’s why I need to know. Would be easier for me.

View Quoted Comment Unfortunately there's no real way to know that this far in advance. It's way too early to expect guests to know if they are attending much less where they plan on staying.

View Quoted Comment Hahaha yes I know it’s too early to know that. But when the time comes, I want to be able to manage this without being behind. I think there’s a website that you can ask questions to your guests. Maybe there I can get that information.

You don't need those details on your Save the Dates, so no need to wait to send them out if you have not yet decided hotel blocks, attire, transport, etc. Hotels often offer free courtesy blocks of 10 rooms where they will hold 10 rooms for your guests under your group code. After a certain deadline, they will release these rooms to the general public if not booked. Some hotels offer discounts. You can additionally ask to hold more rooms, but then you may be liable to pay for those rooms if not booked. Best to go with the courtesy to start and update if needed closer to date.

As a guest, the discount doesn't make or break my decision to stay if the difference is $20-50. But, I'm also the type to book right away, and let my hosts not worry about this. Final payment is when the guest checks out so any price adjustments can be applied.