I don't believe there are any limits with the software, but there are limits when you start talking about a computers amount of RAM.

As far as panning & zooming, you've got the merge plan down, don't forget about reducing the wireframe resolution, that will help. And, with a lot objects in a scene, once I have a group of objects set in the scene where they are supposed to be, I will go into the Attributes and select "Show As Box". This will help a lot with reducing the amount of wireframes that need to be processsed in your gui.

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