Like all GMs, I like to run mystery games. Actually, my horror games often do start as mystery one, as well as most of my fantasy ones. Like all GMs, I do have a problem with the clues: How many clues are enough?
There is no clear and definite answer to that, at least not that I'm aware of. One great solution came from the blog "The Alexandrian", which stated that for every bit of information that the players should find, there should be three clues: 1 that they'll miss, another that they'll understand poorly, and the third for them to understand and to add to their tally.
Another solution came from the "Gumshoe" system, in which finding the clues is not the challenge, but a commodity. The clues that they must have are given to them (if they have the right skill, that is) automatically, with using skills for extra info, that may help, but won't be necessary.
I'm somewhere else, a little bit closer to the one Gumshoe uses, although I used a lot the three clues model. For me, finding the info is not a problem, whether they succeed with the roll or not, the clue will be there, in one way or the other, but there is no one solution to the mystery. In my games, the players, using the clues they have, give me the next clues to give them (based on their assumptions and theories), or in other words, giving me the solution for the mystery.