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There is no limit to the number of planets except your computing power. However, orbital simulations are notoriously expensive for systems with many objects due to the fact that the n-body problem does not scale linearly with the number of objects (in simple implementations such as G3D, the cost scales with the square of the number of objects). In my experience, G3D does pretty well with up to about 500 objects, and my computer is not special.
One of the optimizations of G3D is how it handles planets with zero mass. Since these objects do not contribute to the gravitational field, their effect on other objects doesn’t need to be calculated, so G3D pulls them into a separate list and handles them specially. One can get a few thousand zero mass objects running pretty smoothly, depending on your graphics card (drawing limitations can become a bottleneck, workarounds to be discussed more later). I used zero mass planets some time ago to do a neat simulation of Lagrange points. Sure enough, many of the objects ended up coalescing in the stable L4 and L5 points, just like asteroids do in the Lagrange points of the Sun and Jupiter!
Wow, it sure would be tedious to enter all those object in, and hence a request I received to allow copying planets. G3D does not support this at the moment but in the mean time, if you are comfortable opening the .g3d files with a text editor, you can cut-and-paste planets in the definition file and edit their values there if you want. This can be a much faster form of entry and an easier way to get, say a 100 planets into the list quickly. If you’re comfortable writing scripts to generate text, many more options open up.