I pitched 1 pack of Safale 05 in my 1.080 beer. That is all the recipe called for but I am afraid it may be an under-pitch. Is it advisable or possible to pitch more, its been fermenting 2 days at 64 degrees.
That yeast is 6x10^9 cells per gram at packaging. 69 billion cells in the whole pack. Your beer needs about 279 billion cells (5 gallon batch)
While 1 pack is an under pitch you should be ok. Some styles do better with a slight under pitch if you want those growth esters.
It's always a good idea to do a starter it will at least proof the yeast for you and you can control the cell count better.
Edit: I always get flack for recommending a starter in most cases. I hold true to this my only exception to not doing a starter is if I'm using fresh trub that I personally harvested. I've seen too many beers go bad from underpitching because viability was assumed. You can't assume viability! Simply because of shipping and vendor mishandling is reason enough to do a starter. Yeast has a thermal death point starting at 130°F. Maybe because I live in the South West USA I'm more cautious, here on a warm day inside of a car can reach 140° in minutes killing pets & children all the time, let alone the back of a delivery van. So do whatever yeast practice you like, I just know better than to assume viability. If a baker will always proof yeast for $1.00 worth of dough, why wouldn't you for $50 worth of ingredient?
Yeast pitching rates aren't always too easy to figure out. No one seems to agree on how many cells you should expect in a gram of yeast (various sources state anything from 6-20 billion per gram), and different yeasts have different size cells and clumping tendencies, so the correct number of cells can vary between different strains of yeast.
For a rough estimate on how much yeast you should pitch, check out this yeast pitching rate calculator. The manufacturer of Safale states there are at least 6 billion viable cells per gram at packaging. The guys over at BrewersFriends.com however have found that a single pack of healthy dry yeast was enough to fully ferment a 1.050 gravity beer (the calculator would tell you to add at least 3 packs if it really was 6 billion/g).
For a 5 gallon batch of beer at 1.080 (which is a pretty big beer), you'll need at least 2 dry yeast packets, properly hydrated. If you aerated the wort sufficiently before pitching the first packet, then you should be able to add the second pack without any extra aeration and save your beer. If not, you'll want to make a starter for the yeast, heavily aerate the starter, and let the yeast finish their initial growth phase before pitching into your beer.
I agree with all of the above. Assuming that you are making a 5gal batch, one packet of US-05 is seriously under-pitching and probably won't make a good beer. However, the question asked whether it's advisable to add more yeast now that the fermentation is underway. The two consequences of under-pitching that you're likely to encounter are ...
1) Increased production of unwanted bi-products of fermentation; particularly fusel alcohols and acetalehyde.
2) Fermentation is more likely to stall.
With this in mind, pitching more yeast can restart a fermentation, and reduce acetalehyde, but it wont do anything for the fusel alcohols. Once primary fermentation is complete (should be about now) take a gravity reading and taste the beer. If you get strong solventy flavours then decide whether you want to cut your losses an dump the batch. If you get a lot of acetaldehyde or the gravity is higher than it should be then it's worth adding more yeast. If it tastes OK and the gravity is where you expect it then you've probably got away with it.
When adding yeast to an active fermentation you should make up a starter of the size that should have pitched initially. Don't wait for the starter to complete fermentation, pitch it when it's at high krausen. That gives the yeast the best chance of thriving in what will already be a high alcohol , high C02 environment.
Contrary to all the theoretical advice, I have some practical, experience based advice....you'll be OK. I've done it many, many times with fine results. 1.080 is about the limit if where I'd feel safe (although I have gone a few points higher). So, in the future you'd probably be safer to pitch more, for now you should be OK.