underpitched o not?

Hi. Guys i recently brewed an imperial stout and the recipe called for a yeast starter or 2 activator packs of wyeast.. the thing is i only used one package of wyeast because the other one got lost somehow. it had an OG of 1.074 going into fermenter.

Two days went by and no krausen or co2. then suddenly had to get a blow off setting cause it just went crazy. Big thick krausen and hasn't stop bubbling for 5 days..

My question is... is this an illusion.. should i pitch more yeast regardless all the activity and follow the recipe? or is it to late to re pitch. tomorrow its gonna be a week since i brewed it.

If it started fermenting I wouldn't worry too much about under-pitching. Somebody once told me that in theory you only need 1 active yeast cell to inoculate a wort. Some would also say you may stress out the yeast you have which could produce some flavors you hadn't intended.
I would check the gravity after a week or two. Maybe, depending on your conditioning and bottling plans, you add more yeast at bottling time, but that's further down the road.

At 1.074 you'll probably be stressing out the yeast cells. It may stop fermenting early and finish the beer at a higher FG. Most likely everything will be fine and your beer will turn out great. Just check your gravities as it goes. The fact that you said its been actively fermenting for 5 day makes me think it will turn out okay.