You can have quite a few items in a room. It's more of an issue about how many polygons are in the items or how large the textures are. Just check the product page to see how large the item file is. The rest depends on who's computer you're useing.
If you've only got a standard video card and 256 megs of ram you won't be able to add tons of stuff. 3D programs like lightwave and 3d Max normaly recomend a minimum of 512 megs for smoothe running.
I run IMVU off of a 350Mhz pc useing standard graphics. I get less than five frames per second.
This computer is old. If you're system is new or up to date you shouldn't have a problem unless you turn all your graphic settings up.
I don't have hard facts and numbers, but there is a limit to how many seats you can add to a scene. That can be hacked though.
There is one scene/locked room that has 70+ seats.
You should open a room and just add one item to it. Note the file size. Then add muliple copies of it.
See how many you can get before it bogs down the animation and movement of the camera.