The number of objects you can have at once is 10,000. If you mean displayed on the screen at once, I'm not aware of any limit imposed on MMF 1.5. Possibly Clickteam would be the best people to ask in this case.
"Now I guess we're... 'Path-E-Tech Management'" -Dilbert
The max is 10000, but it's automatically set to 300. You can change it in the frame properties. Yeah, it applies to just about all non-backdrop objects (arrays, actives, etc). There's no limit for backdrop objects, since they don't use any memory.
I think the 'different' objects thing was only in the old MMF. The 1.5 installer (or was it the manual?) mentioned deleting that MMF application that allows you to limit the extensions loaded.
Or was that just for loaded extensions, instead of used ones?
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.