It all depends on how big your stock bowl is. Usually stock bowls are small clay egyptian versions. For a bowl that small you only need one coal. If by chance that doen't give you enough, then you coud always add another half and observe. If you buy a large bowl or a Tangier's phunnel bowl, then that's when you have to start off with two or more depending on size.
Experimenting is truly the only way for you to find out how many coals you need for your specific bowl and shisha. I advise to start off with one and then take it from there.