It is like any other architectural choice - there are pros and cons both directions. If you have separate db instances, you have to scale up the operations to manage each one - migrations, scripts, etc need to be either run against them all, or you need good tooling in place to automate it. A single instance avoids all that, but is more complex in the actual software and definitely more complex for security. A single DB also would let you share data amongst organizations fairly easily, but whether that is good or bad depends on your product. I've created and run products both ways, and I like separate DBs at small scales, single DBs at medium scale, but separate DBs again at huge scale if you also put management tooling in place.