. . . what do you have to do if you, say purchase another gun and want carry it?
You can modify your permit. You have to qualify with every gun on your permit, so you'd need to qualify with your new gun (attend a course, and show sufficient aptitude with the gun). Then take the qualification certificate to the sheriff's office (uhh, don't carry at the sheriff's office ) and make the changes on your permit. It's a process, but very doable. I only needed to do it once. I've been happy with the three on my permit for a long time now.