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

You need enough host RAM to cover all 20 device allocations plus your operating system. Genymotion says each device memory size must stay below your computer’s memory, and it recommends 512 MB to 2048 MB per device for best results.

So, 20 devices at the recommended range means roughly 10 GB to 40 GB just for the virtual devices, before adding your host OS and other apps. Virtual Devices Settings

How much RAM does your host machine currently have, and are these 20 devices meant to run at the same time?