Is that 20 RHEL VMs on one VMware host? Patching an OS generally means you have to reboot the system which implies downtime and loss of service to your customers. If you went ahead and patched 100 servers all at once and the patch failed, what's your back out plan? How big a disaster to your company would that be?
Are all the 20 RHEL VMs EXACTLY the same? Same version of Oracle (or whatever database you may have) Same applications on them all? I rather think not. To remediate the risk you need to gather all the relevant information on EACH VM and check it against the patch. This probably includes checking against firmware patch levels for Array controllers, BIOS, nics, HBAs and disks on the host server. Pain huh?
Prove the patch works against a development or test system first, then I'd patch the DR system (preferably before a DR fail over test) then if when you fail the production system over to the patched DR system, if you have problems, you can just fail it back again.
In major commercial companies, patching a live system is a real pain in the aspidistra as you have to get all the resources needed in place; SysAdmins, DBAs, Application Analysts and business owners plus you need to include backout plans for when it doesn't go as it should. You did run a system backup before you started didn't you? That's one per VM plus the host server if you're patching it as well!
Patching a DR system (not live, no mounted databases, no disruption to services when rebooted.) needs less resources so is easier to manage. All this is to manage the risk involved in patching a system.
The problem is bigger than just setting something up to automatically patch a load of servers whether VMs or bare metal.
Anyway... that's my thoughts for what they're worth.