Well, the "It all depends" card has to be played here.
For Hunters, there may be a processing issue when playing with more than 4 people. For Tetris on the other hand, clearly having more than 4 people shouldn't be an issue. In fact you can play up to 10 on the local wireless. So why not 10 player matches online? Bandwidth? the local wireless is over the same channels (figures in the same ballpark anyways). Latency? Not that critical that your view of the other players be 100% accurate. So what gives? IMO, it's they way they handled the matchmaking. The software has a hard enough time matching up 4 people, can you imagine 10?
Personally the matchmaking software they use is really lousy. It's almost like gamespy was running it or something.
It could be a lot better though, and since it all goes on at their servers in the sky, it's possible that current games could see an improvement later on (although your not going to magically get 10 player tetris online, well not unless someone actually does the DS tunnel thing)