Fhillipo,
First of all the ZigBee specification specifies that the maximum depth is 15.
(Look for nwkcMaxDepth in the spec), but the Stack profile 1 that specifies what is implemented and tested specifies a depth of 5.
If you need to move beyond this you should consider using a 802.15.4 Beacon mode tree network.
Br,
Mads Westergreen