What you need depends on the situation.

I use a pair of muffs that fit over my ears comfortably that have a 35 decibal sound reduction. For the outdoor range I use it's enough most days. I've used them with no problems when a couple of older gentlemen were sighting in their African guns. They were sighting in a couple of .375s. I could hear them firing, but had no ringing in my ears. The closer guy was about 15' away from me, and the other guy was 30' away from me.

Although, if they were shooting those guns again I'd move further away. You could feel it when they fired.