You almost never need more than one prospector active. You only need a prospector limpet on the asteroid you're currently working on.
It's true that prospectors sometimes fail to launch or, if you're firing them on the move, will collide with your ship or something else and die, but once it's attached to the asteroid, it's basically done its job.
You then get the "prospector limpet failed" message when you fire another prospector on some other asteroid, or if you go beyond the max distance of the prospector controller.
I always equip the smallest PLC with only 1 active limpet. There are border cases where you could theoretically use two prospectors at the same time, for example you could work on two adjacent asteroids at the same time which could speed things up during laser mining, but most of the time you only need one.
As for the "enhanced collection" - the bonus in extra materials and ore that the prospectors give is tied to the asteroid they're attached to, not the refinery or the collectors. It means that when a prospector is attached to the asteroid, you simply "see more stuff in it" and can mine it out of there.