if you want 10 first items in an array you should use slice
eventArtist = item.artists.artist.slice(0, 9);
I updated the jsfiddle : http://jsfiddle.net/SBvUm/3/
In order to control that "toString" array representation better, you can use "join".
For example : to get a space after each comma, your final code should look like this
eventArtist = item.artists.artist.slice(0, 9).join(", ");
The problem you have is that sometimes item.artists.artist is not an array but a string. In this case, slice and join will not work properly.
Your code should look like this
updated the fiddle : http://jsfiddle.net/SBvUm/7/
var eventArtist = item.artists.artist;
if ( typeof(eventArtist["join"]) == "function" ){
eventArtist = eventArtist.slice(0,9).join(", ");
}