Somewhere in triple digits; I didn't have a set number. I know there are good players who may be under-ranked due to not traveling, but that's true in the US, too (I can think of many; visualize where Escher would be ranked if he traveled like Trent). Without knowing which continent has more such players, we can't say what the impact is.
Part of the point of any ranking system is that to be highly ranked, you have to go out and prove yourself against quality competition on a regular basis at varying locales. Golf, tennis, etc. won't rank you in the top ten just because you make the semifinals at the US Open every year. Can you beat different people on different courts / courses under different conditions? And are you willing to go out and do so? To be highly world-ranked, a player should get out to see at least a decent share of it.
I suspect that NES is still one of the 10 best pinballers on the planet, but he chooses not to play tournaments these days, so I don't have a problem with his not being ranked there. [Neil, we miss you!]
From a statistical perspective, one-and-done leaves for more room for good players to have a bad run and moderate players to have a good run. More qualifying games gives the law of large numbers time to operate and have the cream rise to the top. But this also partially assumes the same number of tries for all players, which Herb doesn't do - - the midlist players often put in more entries than the top 25 types. That's why I think the "N total tries across Y games" model, where N is at least 5 times Y, is the most likely to properly rank the qualifiers. I choose the option to play different machines a different number of times, rather than a fixed Z games per machine, since not all players can be familiar with every game; this allows them to play the ones they rarely practice on more to compensate.
No model is best for all purposes: one-and-done handles large crowds and limited time better; Herb builds prize pools better; X-Y ranks qualifiers better. It's all up to the event's constraints and the TDs' goals.