Identifying the exact number of dangerous or poisonous fish species is complex, as it depends on the criteria used for classification (such as toxicity levels, types of toxins, and the potential for harm to humans). However, there are several well-known categories of fish that pose risks due to their toxicity or potential for causing illness:

1. Pufferfish (Tetraodontidae Family): These are perhaps the most famous poisonous fish, known for containing tetrodotoxin, a potent neurotoxin. The preparation of pufferfish as food (fugu in Japan) requires highly trained chefs due to the extreme risk of poisoning.

2. Ciguatera Poisoning Fish: Many fish can cause ciguatera poisoning, a type of food poisoning caused by eating tropical reef fish that have accumulated toxins produced by certain marine algae. Common species associated with ciguatera include barracuda, grouper, moray eels, and snapper.

3. Scombroid Poisoning Fish: This type of poisoning is due to the ingestion of improperly stored or spoiled fish like tuna, mackerel, and mahi-mahi. It's caused by high levels of histamine and other biogenic amines.

4. Red Tide Poisoning (Shellfish): While not fish, it's worth mentioning that certain shellfish can accumulate toxins during algal blooms (red tides), leading to paralytic shellfish poisoning, neurotoxic shellfish poisoning, and other conditions.

5. Other Species: There are other fish species that can be dangerous due to toxins. For example, some species of lionfish have venomous spines, and certain species of surgeonfish can cause ciguatera poisoning.

Given the wide range of species and the variations in how toxins can be accumulated or produced, it's challenging to provide an exact number. However, the number of species that are commonly recognized as posing a risk of serious poisoning to humans is relatively limited, likely numbering in the dozens. It's important to note that in many cases, the danger they pose can be mitigated through proper food handling and preparation techniques.