How many pyrotechnic vdss are to be onboard boat?

This would depend greatly on the size of your vessel and where you are operating your vessel at. In the US you can often use the Code of Federal Regulations (CFR's) to find out what you need. For pleasure craft it MAY NOT be required to have aboard, however it is ALWAYS better to have and not need to, than the other way around. Checking with your local Coast Guard unit, State Police or local Boating/ Marine supply store is always a good bet for accurate info.