There are no numbers to tell you, but a few facts:
1) The total number of objects on a layout isn't important. The total number of objects on screen is. So, if most of the objects gather around one spot on the layout you will reach a limit much quicker than if they are spreaded around the layout.
2) The window size counts. Imagine Full HD against VGA. Not only does the graphic card have to render 6.75x the pixel count of VGA, the viewable area is also larger and therefore more objects are visible at once (see point 1)
3) The editor has a hard time handling the selection of many objects/instances. Avoid it as much as possible.
4) The editor will have a hard time handling a huge project. This is why it may be better to use C2 instead. However, the runtime is rock solid, so if you can manage to work with a sensible editor that leaves you waiting 30 minutes when opening a huge project, crashes a lot and has its hiccups, then you get a stable and fast game when compiled (better: if compiled <img src="smileys/smiley2.gif" border="0" align="middle" /> )