Ok but is-it possible to use only once each bitmap ?
No, is not possible to do it automatically. You would to change Forest to Custom Edit mode and edit the IDs of the duplicated items, but this is a manual process.
I have to make a crowd whith 50 bitmaps of people, and i don't want to see twice the same bitmap. Should I create as many item as i have bitmap manually ?
Yes, but i just created a script that may help you:
To install the script:
1. Copy the MultiSubToForest.ms file to <maxdir>\scripts\startup
2. Run 3ds max
3. Create a new toolbar (Customize User Interface), and drag the macroscript MultiToFO3 from "Itoo Software" category to the toolbar.
To use it:
1. Create a multi-sub material containing all bitmaps
2. Create a box and assign the multi-sub to it
3. With the box selected, run the script "MultiToFO3"
4. It will create an empty Forest 3 with the Geometry List filled, one item by each material in the multi-sub
Also remember that you can create and organize your own items in the Library Browser, defining materials, sizes and other properties.