This is subjective question. Requirements for each VM type and use will vary.
If you are modeling an access network with mostly vIOS devices you can get a fairly high node count.
If you are modeling a provider network with XR and vMX devices then not so many.
Avoid swapping if possible. Use of swap space leads to startup issues with VMs such as the Cisco XRv9k.
Most manufactureres lizst their requeirements , for example
https://www.juniper.net/documentation/e ... imums.html
Your hosting platform will also contribute to determining memory resources used. I mostly use KVM, so the memory allocated to a given VM is not alway held resident. A cisco XRv9K 6.4.1 will not start properly with less than 16 G allocated, however it may only use 8-11G when running.
sks: 1240 total, 10 running, 1230 sleeping, 0 stopped, 0 zombie
%Cpu(s): 76.8 us, 18.9 sy, 0.0 ni, 4.2 id, 0.0 wa, 0.0 hi, 0.1 si, 0.0 st
KiB Mem : 26412465+total, 5543180 free, 24426953+used, 14311940 buff/cache
KiB Swap: 37728252 total, 17334880 free, 20393372 used. 14851840 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
20490 root 20 0 17.648g 8.386g 13708 S 466.6 3.3 40:52.68 qemu-system-x86
57912 root 20 0 17.315g 0.010t 13984 S 303.6 4.2 25:38.91 qemu-system-x86
54202 root 20 0 17.263g 9.822g 14104 S 287.5 3.9 23:28.59 qemu-system-x86
27271 root 20 0 9777244 6.999g 13604 S 280.3 2.8 20:48.45 qemu-system-x86
A network with ~ 40 high test devices ( 80% XRv9K , 20% vMX) on an 80 core/thread box with 256G of ram