Counterpoint may leave too much implied with only two voices; with four or more voices one must increasingly break or relax various rules that promote voice independence, e.g. the use of parallel motion where additional voices simply double some other line (they can't all be independent, there's too bleeding many of them!), or to drop voices for a thinner texture, for example where there are five instruments but only three or four of them are sounding together most of the time. That's a long way to say that around three to four voices is ideal if you want independent lines (except they're not really independent, like two people shouting past one another; there's a weird mix of both working together while each yet manages to stand out in good counterpoint) though even better than this claim would be to compare, say, Bach's two-part inventions to works that have more voices.

For those who do not know counterpoint, you have only three motions a voice (a horizontal line of music, traditionally sung) can make relative to another voice (move closer, apart, or to hold steady) combined with limited voice ranges (say, a doubling of frequency, or so) and limited interval choices (seven, or so) within an octave or frequency doubling, and the voices are very close to one another but only rarely cross one another, on top of all that various rules systems that forbid or frown on such things as the tritone, parallel fifths, and so on into the weeds such that with more than a few voices you quickly run out of valid options for all the voices to move independently.