Up to this point, I've had every property on its own lock, and I keep a copy of all of these keys in a box. What systems are you using that are working for you? I'd like to systematize this now that I'm accumulating more properties and It's becoming harder to
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I put replaceable-cylinder locks from landlordlocks.com in my rental. They cost more up front, but I can re-key the whole house myself in 20 minutes or so - you order (or buy ahead of time) a new set of cylinders, and use a tool you get with the new locks that removes the cylinder and installs the new one. They will also set up a master key / tenant key system for you, so you can have one key that gets you into all of your properties, but each tenant's key only gets them into their unit.
A couple of the locks took me a little longer to install in the door than "regular" Kwikset/Schlage/etc locks from the hardware store, but that's because I decided to install a couple of extra screws that the "regular" locks don't usually have. You should only have to do that work one time, though. One of the locks got a little stiff after I used it for a while during rehab; a local locksmith recommended and sold me a can of "Tri Flow" spray lubricant, and that cleared it up.
Most hardware stores won't duplicate the keys for commercial locks like this. A local locksmith might, but you might have to show some proof that you're the owner of the property.
I am not affiliated with Landlord Locks, except as a customer.