This is why I believe ChrisC will get to that 400 ski area mark.
I am not sure about this. I have generally skied the worthwhile areas in some of the denser ski resort regions in North America (New England, Mid-Atlantic, Northwest, CA, Rockies), and there are not many worthwhile mountains with 1,000-1,500 vertical feet of elevation.
In the Alps, I have often returned to my favorite areas to ski more off-piste lines/zones/itineraries in good snow, rather than trying out mid-sized resorts (that would be major resorts in North America). Places where I have had three visits include: Chamonix, Courmayeur, Val d'Isère, Serre Chevalier, Les 2 Alpes, Verbier, Zermatt, Cervinia, and Andermatt. There are plenty of ski areas nearby each of these (the Alps are so densely packed with ski areas), to boost numbers.
However, there are still many interesting places in Europe to explore, such as La Clusaz, numerous locations in the Swiss Valais, Austria's Vorarlberg/Tyrol, and more. If one were to add significant numbers, it would be Europe.
Likely, I can visit some New Zealand ski areas. Perhaps consider returning to Japan and adding a few more in the Nagano area. Still, I'd rather go back to Hokkaido to ski Niseko, Rusutsu, plus some new ones like Furano, Asahidake, and the Otaro areas.
Personally, I am interested in skiing in some areas in British Columbia in the future.
If I really wanted to max out ski area visits, I would have skied the Arctic resorts of Finland (Levi and Ylläs are 30 minutes apart). I did not because Nordic countries are expensive, require flights, have extensive but Midwest-like skiing, and are dark, requiring night skiing (10-3 twilight/no real daylight), and I have already seen the Northern Lights. Switzerland is cheaper.
Levi
Even with the season-opening World Cup Slalom event being held here, Levi is still quite small on a world scale. Its comparatively miniscule 335m of skiable vertical is served by a nearly ridiculous 26 lifts, accessing a respectable 45km(ish) of ski trails (incl. ski routes). Piste quality is superb however, and the snow parks are allegedly first class.
Ylläs
Finland’s largest ski resort, Ylläs, has the longest runs (over 3km), the highest skiable vertical (464m), the highest lifted point (719m above sea level), and the most piste terrain (53km) in the country.