Is there any limit of objects (trees, rocks, bushes, etc...) loaded using PagedGeometry?.
Is it a good idea to preload the whole objects in a huge map at the beginning?.
There aren't really any hard limitations, so it all depends on how much your computer can handle. The only real limit is the amount of variety of objects, because they take memory, and need to be loaded individually. Usually the lower number of tree types, etc. that are grouped in a certain area the better, for performance.
You can theoretically have millions, or even billions, of trees, bushes, grass, etc. just as long as the viewing range and LOD levels aren't too much for your computer (like maybe only ~50,000 on the screen at any time). Of course this depends on your computer too, so it helps to experiment.