subscriptions limit - free tier

I created a virtual lab environement for exam preparation. This lab is pulled up using Vagrant and VirtualBox. At the moment there are 6 VM-s in the lab, all RHEL9 images, registered using "subscription-manager register ...". Now there are some repos enabled through subscription-manager, working just fine.

Sometimes I make sime mistake and want to throw away the whole lab env and create from scratch using "vagrant destroy ..." and "vagrant up ..."

My question are as follows
* what is the correct method to get rid of the VM-s I am destroying? Do I need to unregister them before destroy, or is it just enough to delete them on the systems inventory page on RHEL website?
* is there a limit in the free trial account on the number of registered RHEL instances?

Bonus question: At the moment my Vagrantfile is configured to run "subscription-manager register --user ... --password ..." on initial machine provisioning. Is there a better way to do it in an automated way?