Hotel blocks

Hotels do offer wedding discount packages to you allowing your guests to book on their end with the rooms requirements of how many needed from you. It would be good to note this on the invitation and website for hotel bookings under your names and date to verify which you can know who has booked. You can help guests to book rooms for them and get their credit card information to put on hold until they arrive to check in. You may want to inquire about the extra rooms not booked if they will have it charged to your card or it will not effect you in any way. The contract would state this too to make sure your aware of the information.

My venue is a hotel so they offered a discount for anyone who wants but there is no penalty if no one books

Is there a penalty if your block is not used up or does it simply get released for public sale? If there's no penalty, it doesn't hurt.

If you're downtown and you're able to get a pretty sweet deal on a room block, then I think it'd be nice for your guests. New regulations in Toronto means only registered AirBNBs are allowed to operate short term rentals based out of the owner's principal residence. Now, whether enforcement will happen is anyone's guess but considering the city's shortfall in revenue from the past two years and people's tendency on snitching, your guests would probably not want to risk the chance for having to book another hotel last minute at a higher rate in the event their AirBNB gets cancelled.

I would ask your guests about what they're doing for accommodations. Sometimes it's cheaper to go with air bnbs if most of them are wanting a hotel then I would set up a block of rooms.
My fiance and I have done that since our entire guest list is traveling to the city we're getting married in and need places to stay. So we picked 4 different hotel for them to choose from depending on how much they wanna spend

I have about 20 guests traveling and I've been asking them what their plans for accommodations are and quite a few said they are gunna book an AirBnB so I figured a hotel block isn't needed