How much memory do I need to have 20 virtual devices?

Genymotion does not give a single fixed RAM total for 20 devices. The device memory is set per virtual device, and the recommended value is usually 512 MB to 2048 MB per device. Virtual Devices Settings

So for 20 virtual devices, that means about 10 GB to 40 GB of RAM for the devices alone, plus memory for your host system. You also need to keep some RAM free for the OS and other apps, because the allocated value must stay below your computer’s available memory. Virtual Devices Settings