You can have up to 9 home screens.
1- Download and install
modified hildon desktop from here. It may take several minutes to download and install packages (5min or so...)
2- Reboot your N900
3- Open a terminal and type the following commands :
Sudo gainroot (then, Enter key)
gconftool -s /apps/osso/hildon-desktop/views/5/bg-image -t string "/path/to/background.jpg"
gconftool -s /apps/osso/hildon-desktop/views/active -t list --list-type int [1,2,3,4,5] (then, Enter key)
4- Done!
With this example you have 5 home screens! Works perfectly. Have fun with up to 9 Home Screens!
Source: http://maemobriefs.blogspot.com/2010/03/how-to-have-up-to-9-home-screens.html