MEmu can run on 768 MB and 1 cpu core per instance.
(MEmu can run on 512 MB too, but it can get a bit laggy if you run for multiple weeks without a reboot)
Nox can run on 512 MB and 1 cpu core per instance.
Bluestacks i usually run at 1 GB per instance with 1 CPU or 2 CPU cores depending if the instance gets laggy or not.
(Bluestacks is in my opinion the bulkier of the three i have mentioned. (because i never used any other emulator))
Of course, the amount of bots you can run on your system depends on your hardware. Also remember, that not ALL resources (CPU and RAM) will be usable for your bots, the windows operating system also takes quite a bit from them.