I believe the limit on the total number of structural stages in VDDT is indeed 99, although I would need to double check that with Leonardo when he returns from vacation at the end of the month. A simple test to confirm this would be to enter a structural stage in a dummy VDDT model with a code greater that 99 and see if it complains. Note that you would need to add a state class to your model that uses this code to properly test VDDT.
Path is also limited by any VDDT limits at runtime, although Path no longer asks the user to provide the numeric codes. However they are assigned at runtime based upon the auto-generated "Structural Stage ID" field you see in the Path definitions. So Path is also limited to a maximum of 99 structural stages.
There is a way to track more than 99 "structural stages" in Path, however, using State Attributes. Under Project|Definitions in Path you could set up a state attribute group (say called "My Structural Stage") and then create a state attribute type associated with this group for every structural stage you wish to track. You then assign each state attribute type to a combination of cover type and structural stage. So for example you could have a single structural stage called "Open", but have 2 state attribute types called ""Forest Open" and "Non-Forest Open". The "Forest open" attribute type would be mapped onto any state classes that have cover type of "Forest" and structural stage of "Open", while the "Non-Forest Open" attribute type would be mapped onto any state classes that have cover type of "Non-Forest" and structural stage of "Open". Doing this has essentially doubled the number of possible combinations for the original structural stage "Open". Note that you would need to assign values of 1 to the State Attributes property for your scenarios to make this work.
One caveat here is that the State Attributes property is new to Path and so it is only tracked through the Path database output – in other words you can’t track these new attributes in the old legacy VDDT output files.
Hope this helps – post again if you have more questions. Colin.