Should I pitch more yeast?

Here's the skinny,
Yesterday I brewed a 5 gallon batch of holiday ale with a gravity of 1.058. I pitched one package of s-04 yeast without rehydrating and this morning the airlock was full of kreausen. My question is this: should I add another package of s-04 to the wort to ensure complete fermentation or will the one package I piched do the trick? From what I read pitching dry yeast without rehydrating kills about half of the viable cells. Any advice is appreciated!

No, you have a good fermentation. Also, people get perfectly fine beers without rehydration. I personally don't bother to rehydrate my dry yeast for beers under 1.060 OG.

Pitching without re-hydration does kill some cells, but a packet of dry yeast has so many cells that it doesn't really matter for lower OG beers. You'll be fine for a 1.058 beer, so no worries. Even if you had underpitched, adding more yeast after fermentation is in full swing won't really fix the problem.