Uh, why has no one here said the blatantly obvious in this thread? This question is basically unanswerable without knowing what exactly you're writing. Four chords is more than enough in your average pop song or quasi-minimalist piece but probably nowhere near enough in a Romantic symphony. More than that, it also depends how often there are harmonic changes. It should be fairly obvious to say that piece with a harmonic rhythm so slow that four chords take four minutes is probably going to need less total chords than a piece with a harmonic rhythm so fast that four chords happen in 5 seconds. Hell, I tend to write whole sections of pieces where it's not even appropriate to really talk about 'chords' so much as it is macroharmony and if you want to reduce that to chords, then I write pieces with only three-four chords. Without a stylistic context, this whole thread just become irrelevant.