I know that most of these multi-barrel pistols have two barrels in the over-and-under or side-by-side configuration and I (very rarely) have even seen some with three barrels.

My question is that are their any other pistols with a greater number (more than three barrels) of barrels (not including volley guns or pistols that fire all the barrels simultaneously) and can fire a bullet independently (single-shot) from each pull of the trigger(s)?

The pepperbox, (if you've played the game Clue it's what they call a revolver) an early predecessor to the true revolver. Rather than a revolving cylinder, it normally had a number of barrels (the average probably being 5-6 although I've seen anywhere from 3-10) grouped together that would be rotated either by hand or by a mechanism to fire a single shot in turn.
Hope this helps,
Dan
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