Slurm is absolutely ubiquitous in the high-performance computing (HPC) community. I believe its only similar competitors in the HPC space are the SGE [1] and Torque/PBS [2] resource schedulers.
I'm not sure of the exact numbers, but I would guess that an overwhelming majority of the Top 500 Supercomputers [3] are running Slurm. And as others have noted, research computing centers in academia all mostly run Slurm. And Slurm also dominates in the DoE national labs in the US.
Oh, and as a [potentially apocryphal] fun fact, the name "Simple Linux Utility for Resource Management (SLURM)" is a backronym from the soda in Futurama! [4]
[1] https://en.wikipedia.org/wiki/Oracle_Grid_Engine