May have short-changed the yeast, should I fix?

Noob brewer here, and I brewed my 3rd batch this weekend. It's an extract kit of Scottish Ale from NB, with a 90 min boil and an OG of 1.064. I have used smack-packs for my previous brews with success and did the same here. But, I have never used a starter before, just the packs as is. The ale's instructions recommended using a stater, but I figured that I'd be fine since I had good results without one for the previous brews.

Well, after reading more about yeast starters I realize that I may be lacking in my yeast count for this batch. I'm getting good fermentaion right now, very active. Had to use the blow-off tube and it's bubling like crazy this morning. But, I'm worried that the yeast count will be anemic for the job and I won't get enough sugars converted. My question is, should I leave it alone and wait to see how long it ferments for, possibly giving it a swirl when it slows down? Or should I add more yeast to the wort? Problem with the latter is that there are no local brew stores in my area, so to add more yeast will require me to order some and wait for it to be delivered, 5 days at a minimum. Is that too late to add more yeast? I'd really hate to have an under-fermented, too sweet of an ale. Thanks.

You'll be ok with the one pack. You said it yourself that you've gotten good results using just the smack pack in your two previous brews.

Next time, make a starter. It's already too late for Brew #3 since it is already actively fermenting. If you were to pitch a fresh pack of yeast, those new yeast would be overwhelmed by the alcohol already present and essentially say "screw this," and flocculate out without doing anything productive.