StatMuse is telling me Westbrook's 18 last year is the most, but I don't think their data goes back far enough. Westbrook's 18 is the most triple doubles in any season since the 83/84 season at least.
Edit: Wiki says
Quote:
Most triple-doubles in a single season: Oscar Robertson recorded 41 triple-doubles during the 1961–62 season.[23] Wilt Chamberlain is in second with 31 triple-doubles in the 1967–68 season.[25]
Given the pace adjustments and how liberal scorekeepers were back then, I'd guess Westbrook's 18 is on the same level of impressiveness as 30-40 from back then.