Personally, a single Flash has always been enough to start off versus an enemy group if you can hit everyone. In cases were the group is semi-divided such that a single Flash wouldn't hit everyone, I'll usually Flash the majority of the enemy group, hit whatever wasn't Flashed with Shield Lob, and then follow up with a single Flash to the whole group for good measure.

I don't know about this 5x Flash thing but that sounds like overkill and a waste of MP, unless it's a strategy for one specific boss fight that I'm not familiar with, or Coil/Extreme stuff.