Answers to your 2 specific questions:
- Assuming a well lit scene, identification roughly maps to 150+ PPM / 50+ PPF. Recognition is approximately 50 - 150 PPM (or 15 PPF - 50 PPF). Detection is below 50 PPM / 15 PPF. Even in ideal conditions, one typically needs ~15 PPM / 5 PPF to detect a subject. (Note: this is for conventional, not thermal cameras).
- PPM / PPF values 'change' based on: amount of light (low light and high contrast require increased pixel density), level of compression (too heavy compression will require increased pixel density), and lensing (out of focus, poor quality lens will require increased pixel density). As for specific vendors, there's no easy relationship between some vendors inherently requiring more than others. However, decisions by the vendor (poorer quality lenses, defaulting to heavy compression, inability to deal with poor lighting) can increase the values needed.
For background, see our PPF / PPM Guide.