We haven't found a hard limit, however keep in mind, the more planets, the worse the performance and it doesn't seem like it's just a linear increase, either. Empire's End has pretty significantly worse performance than Art of War, with just 20 more planets, so the more you scale that, especially as groups get added, the higher the requirements would get for CPU strength as you go. This is especially problematic because CPU improvements tend to be in number of cores, whereas EaW is a 32 bit game and therefore can only use one core.