My snap reaction is that I don't have one.
But that's not entirely true. My baseline is always what I like, what speaks to me -- whether or not it fits my tastes, past or present (surprises are fun). But I do take into account whether a knife fills an unfilled niche, directly steps on the specs of another knife, or otherwise overlaps too much with my collection.
Because if two knives are
too similar (by whatever nebulous definition I'm using at the time), one of them probably won't get carried. (This tends to keep me from buying doubles of knives I like, too.) I carry and use all of my knives, so that's kind of my soft "cap" on just indefinitely expanding my collection.
Or at least that's the theory.