How many Diabolic Tutors is too many?
There are two basic reasons to use a tutor:

1) You have a card that you want more than 4 of, but there is no similar substitute available.

2) To be able to have a 'toolbox' of 1-of answers to different types of threats.

So what is the card in your deck that you wish you could have more than 4 of?  What are possible substitutes for it in the format you're building for?  Is using a turn and extra mana to get at it preferable to the substitute card?  The primary reason not to put in a tutor is if your deck can reliably deliver on its designed win condition without needing access to more than 4 of a particular card.

Toolbox decks are usually meta-game haterators, so it's tough to talk about them in the abstract.  Mono-black isn't usually known for it though.  Toolbox tutors are more often things like Birthing Pod searching up creatures with desirable enters-the-battlefield effects.

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Diabolic Tutor specifically, your biggest challenge on that one is how it impacts your mana curve.  What are the 5-drop cards in your deck you're tutoring for that you must hit on T5 in order to win?  What 4-drop card are you giving up the opportunity to play on curve in order to tutor for something that comes out next turn?

Turn 4 is not a good time to be making a play that doesn't impact the board state.  Turn 4 is the kill turn for aggro decks in Standard/Modern that got a half-decent starting hand.  Tapping out for a tutor also communicates a lot of information to your opponent.  e.g., The only reason you tapped out for 4 in mono-black standard and played a tutor instead of an Obliterator, was to go tutor for an Obliterator.  They will do everything they can to be ready for it next turn! 

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In decks for big multiplayer games, go for it, play all the tutors you can.  When games regularly go over 10 turns, getting what you need late is more important than playing on-curve early.