A couple years ago, I tried to book 4 connecting roundtrips (i.e. 16 segments) on one ticket to reduce the taxes. The phone CSRs tried hard to make it work with 4 roundtrips (i.e. 8 segments), then with 3 (12 segs), but in the end could only get 2 roundtrips onto each ticket (which still saved taxes vs. buying 4 individual round-trip tickets).

So I'd say the limit is somewhere between 8 and 12 segments on a single ticket.