iPhone servers tend to be made more efficiently than their PC counterparts. iClassic averages around 1000-1500 players and has MOST of its scripts running on the serverside because older generation iPod's can't handle too much going on the clientside. The server can get laggy at times, but it's mostly when new large files(like tilesets) and introduced and a thousand+ players are downloading it at once. Also ALL the players being in a single level can lag the server quite a bit too. There was once a rogue script warping all players to Graal City if they lagged, and because all the players were getting warped there the server lagged and started warping ALL players to Graal City... it snowballed into a big mess. As far as I know complicated projectile stuff is handled on the clientside so it shouldn't stress the servers out too much.
TL:DR? If iZone is on a decent machine it should be fine, and if it's not Stefan will upgrade the machine like he has done for iClassic and iEra.
The only actual server crash I know of due to playercount was when iClassic first hit 1024 players and the gserver was never coded to handle more than that many players and literally crashed because of it.