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.
must be pistol-launched
The minimum number of pyrotechnic Visual Distress Signals (VDSs) required on board varies by jurisdiction. In the United States, for example, the minimum requirement is typically three pyrotechnic VDSs, which may consist of either three hand-held devices or a combination of hand-held and aerial signals. It's important to check your local regulations to determine the exact requirements for your area.