I have a powerful machine with 70 GB RAM. How many Oracle instances with 20 GB can be created?
Your kernel parameters need modifying.

Edit /etc/sysctl.conf and ensure the following lines are present:

kernel.shmall = 18350080
kernel.shmmax = 75161927680
kernel.shmmni = 4096
kernel.sem = 250 32000 100 128


Then reboot the machine.