Unlimited numbers, but like in this example,
$30 to 1.98 million, What?
It was just 6 legged parlay, but still it won him millions with just a small bet, because the odds is +6,576,031. But you can have 10 legged parlay that won't have that kind of odds. So it doesn't matter how many you can put in a parlay.
All depends on the incredible odds, specially if they are all underdog.
It is clear that everything depends on the odds and 10 outcomes with odds of 1.1 will give lower odds than 2 outcomes with odds of 10. But there is an interesting correlation: the fewer outcomes, the smaller the bookmaker's margin, i.e., if you are choosing between a multiple bet with 100 outcomes and a bet with 6 outcomes (the final odds are the same), then you should choose the latter. And the example you refer to confirms this, usually, incredible winnings come from such multi-bets, and not from bets with 100 outcomes.