You asked if the hospital will know they need you.
The float staff is the first canceled if the census is low. You will get a schedule that covers four weeks. You should be asked ahead, let them know days you won't be available, or what days work best for you. The schedule will have what days you are considered available, 2 - 4 - 10 days a pay period what ever you sign up for, and you will be expected to work those days when they call you. The staffing person/nurse will call you (depending on what shift you work) from 2 - 4 hours in advance of that shift telling you where you will be working. They also will call you 2 - 4 hours in advance saying, "we don't need you today." Where I worked it was fine for the float staff to call the staffing person a few hours ahead of shift to ask if or where they would be working.
A very good question to ask when you apply is a rough estimate, guess, of how often you will be cancelled. No one has a crystal ball but they should give you some idea.
Nurses either HATE floating, or are OK with it, or LOVE it. I LOVE floating.
These days with jobs hard to find give it a try, don't let the nurses who hate
it scare you away. I floated as a new grad and still float 28 years later.
All-Nurses needs to start an I Love to Float section.