I'd never join any server other than 16-18 players, unless it was an emergency. It's clearly the range the game works at, on public servers at least.
I've occasionally joined a 24+ player server for a laugh when I wanted to completely walk over some aliens, but it's not terribly satisfying <!--emo&???--><img src='http://www.unknownworlds.com/forums/html/emoticons/confused.gif' border='0' style='vertical-align:middle' alt='confused.gif'><!--endemo-->
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I've occasionally joined a 24+ player server for a laugh when I wanted to completely walk over some aliens, but it's not terribly satisfying <!--emo&???--><img src='http://www.unknownworlds.com/forums/html/emoticons/confused.gif' border='0' style='vertical-align:middle' alt='confused.gif'><!--endemo-->