When there are X pieces of scenery and you add another piece, it will not be rotatable. Also, when you try to rotate said piece of scenery and it is not rotating, the first piece of scenery that was put on the map will start rotating. If you continue to add pieces of scenery, and try to rotate them, you will actually be rotating the 2nd then the 3rd then the 4th, and so on, pieces of scenery ever to be put on the map.

There must be a limit to the ammount of scenery on a map, that you can rotate.

Yes, and I believe it's 255. Look in the developer's section for Mapmaker Plus which will not only add many new features, but also reaise the limit on both scenery and polygons.

Edit: On a side note, it's generally not a good idea to shove all that scenery in the map because it'll create a huge lag problem for anyone playing.