Supervisors...

I am not sure if this is going to be globally relevant, but I need the advice so I hope it is.

I am preparing to apply for phd programs next year, and I am planning on applying to quite a few unis to cover all my bases. In my country (Australia), you need to secure a supervisor before you apply for the program, and attach proof you have organised a supervisor in with your application (I am unsure if this is the same for all countries).

Does that mean that I have to line myself up to be supervised by a whole heap of academics at different unis and then let them all know "hey I'm going somewhere else now sorry and thank you anyway!". Or is it designed for you to only apply to one single program? I feel surely they are not expecting people to only apply at one uni considering how competitive stipends can be, but it also feels strange to fully get approval to be supervised by someone you mostly likely will not end up with?

Help please, I am confused.