See: viewtopic.php?f=24&t=3641It appears that a maximum of 16 new industries can be added with this patch in order to leave all existing industries intact. There is a possibility this could be increased to 18 if replacing the "Dummy" "XXX" building/ruler and "Customs House" will not cause problems.
Also, I think (not sure) Ned coded Trainmaster to use up all available industry slots with its default buildings, so between those two examples you should know the limit for industries.