First off, welcome to the forums!!
Secondly, the limit of 3D models you can have is 65535. Now that will take A LONG time to load, even if it were low poly. You can however have Dozens of models at once all doing entirly different things Check out(
http://www.geocities.com/flyingmonkeyc/Sins_of_the_Moon.html)
) there I have 10 high poly models, and around 30 pickups, and there really isn't a slowing factor(62FPS on DBC!! Just SetSync 0 for optimm speed)
As for 2D, oh yeah you can have THOUSNADS of sprites moveing in all directions with no problem at all, just check out the FAST 2D in the extras file of DBC. No slowin down there,lol...
--carlitos