Hi Yannfr,
Sorry that this thread had gone unanswered for a while-- it slipped thru some cracks.
Anyway, to answer your inquiries (and I'm assuming you're using 3dsmax):
The 8-cpu limit for DBR is indeed posed by the mental ray renderer. Search the 3dsmax User Reference documentation for "DBR", for details.
We've looked into the "split scan lines" rendering method that backburner offers, but it turns out that it's a backburner-specific feature that's not at all exposed as a maxscript command or an API routine, so it is not possible for us to support it, unfortunately.
If your studio has some development expertise, or if you can farm out such work, it should be possible to implement a similar functionality that works via qube, but that would require some non-trivial efforts, and is probably beyond the scope of this thread...
cheers,
-shinya.