Here are two real examples.
1. The dedicated server Intel Xeon E3-1240 3.4 GHz 4 cores, 16Gb RAM, 220 Gb SSD is carrying about 800 devices (production mode, no logging), and this uses less than 10% of the server resources.
Such server should be able to carry at least 8K mobile devices.
2. The virtual machine 2 x 3.6 GHz CPU, 8 Gb RAM and 30 Gb HDD is carrying about 200 devices in production mode within more than 2 years (large database history), and this uses about 40% of the server resources.
Such virtual machine should be able to carry about 500 mobile devices.