There are two major things to think about. Storage IO capacity and CPU load. If you try and push the disks to hard performance will be impacted. Same goes for the CPU.
If lots of users will be logging in at the same time, you’ll need some very fast disks to compensate for the boot storm that happens. If the users will be using high CPU applications like Photoshop or something then you’ll need a lot more physical servers to ensure that there are enough CPU resources available.