You can partition a large compute cluster into many smaller ones. Users can make a request specifying how many processors they want for how long. Check out this link to see the activity of a supercomputer at Argonne.

https://status.alcf.anl.gov/theta/activity

And I believe it is more efficient to have a single large cluster. As there are large overheard costs of power, cooling, and having a physical space to put the machine in. Plus a personnel cost to maintain the machines.