There are some hints in the documentation but it can largely only be
hints as it depends on how that instance is going to be used.

The details will depend on:

0: how many workers are connected to each master
1: how many devices are connected to each worker
2: how many users / expected number of test jobs per day/hour etc.
3: what kinds of devices need to be supported - devices which can only
be tested over USB will impact on your choice as many servers only
have 1 or 2 USB host ports and USB hubs are problematic, even when
powered.