First, do your homework. How many people are likely to attend, how many of these will stay at a hotel (as opposed to students staying in dorms, say)? Set up different scenarios, for low, medium and high attendance.
Will you require anything else from the hotels? You sound like the conference itself will take place at the university, but if you were also going to book conference rooms at the hotel, that would make a difference. Or perhaps you plan on having a cocktail reception at the hotel?
Think about possible time slots for the conference. Any flexibility here can help. Hotels have busy and less busy times and are keen on filling their rooms for less busy periods. Do you plan on people only staying for the conference, or might they want to stay a few days longer (e.g., for collaboration at your university, or for tourism)?
How far out are you planning the conference for? When will people likely finalize their plans? The hotel will likely keep special rates for attendees only until some time before the conference.
In parallel, find out whether the university has an events team that helps organizers of conferences. You are probably not the only person to organize something at that university, and chances are that there are people that do stuff like this all day long. The universities may already have preferred hotels for precisely this kind of opportunity.
Once you have thought about all these points and have rough answers, call hotels and have a talk with their team. They will have done this multiple times before and can walk you through the process. They will likely require answers to questions like these to give you a quote. Get that for multiple hotels, and compare. It might make sense to reserve blocks at multiple different hotels at different price points.
