Alpacas are herd animals and need to be in a group to be happy. Alpacas also need a guard animal in their pen such as a llama or donkey. They are a wee bit small/shy to fight off predators by themselves.

I thought they were guard animals or is it just llamas?

Alpacas can be added to a flock of sheep as leaders (less likely to flock to somewhere stupid), but they aren't as effective at fighting off predators as Llamas are, just like modern improved goats are far more effective at fighting off predators than modern improved sheep.

You will have problems if you're only getting one animal. Replacing the herd with human attention will only make the situation worse. People have been injured from llamas thinking there was nothing wrong with kicking, biting, mounting their caregivers. A healthy distance is what you want and a small herd of llamas or alpacas is ideal. Keep in mind that with a lot of them, what they think is a close knit herd is different than what we think of as a community. As long as they can see another llama they are part of the same group - large distance means nothing to them.

Mounting their caregivers..hmm, that explains why my one neighbor has only 1 llama! :C

Llamas are quite a bit bigger than alpacas I think, and do guard the flock. The males get huge fighting teeth that are usually removed for safe handling. They also have marketable fleece. They are ridden and used as pack animals in South America. I think alpacas might be a bit small for that and are small for riding anywhere, but perhaps the fleece production is good for the food intake in a smaller animal.

I would love to have a big llama!

Do they need another llama or would they bond with and be a pack mate for a goat or sheep?

Here's some llama info sites:
http://home.att.net/~lostcreekllamas/basiccare.html#anchorcanine

http://www.krmllamas.com/llama_care.htm

http://sunriseranchonline.com/uploads/Llama_Care.pdf

http://www.nose-n-toes.com/carefeed.htm

I no longer raise llamas and I'm sure someone more knowledgeable than me will chime in and offer better advice but - llamas work best when kept in groups of other llamas. Sometimes they bond with sheep or goats but sometimes they don't. Some of them are friendly with humans while others tend to stay outside of arm's reach. This is an animal that as adults it doesn't ever touch another llama unless it has to. They aren't into the mutual grooming stuff like horses or goats, they are more like sheep in that they stand a few feet away from each other most of the time. They are so cuddly looking that a lot of people snuggle them (easy to do with young llamas). Then they keep encouraging the llama to "show me love" as it grows up. This can lead to big problems when it hits full adult size and sees you as an equal or potential mate (!). They solve social problems with other llamas with a lot of chest butting and face spitting (which should really be called vomit since that is what it is!). Tame them down too much and they will do all of these nasty things with you too. Then they're deemed a corrupted animal, a problem, a danger... none of this would happen if people would simply keep herd animals in herds with the same type of animal. Unfortunately there are plenty of single llamas out there needing homes.

I agree with trianglejohn. I've had llamas for ten years and raise them on 170 acres- there is a family group of 12 at the moment and they really enjoy one another, they are very social and I love to watch them interact. I've lost one to a mountain lion last year and two to neighborhood dogs- all were full grown at the time. I consider myself responsible for their protection against these types of threats, but they were either alone in a more remote part of the ranch due to social dynamics, or I was out during the day (when the dogs came).

Anyway, I guess what I wanted to comment on was that in general you need more than one llamas for them to be happy UNLESS they are guard llamas (not all can do this job) and are bonded to their flock. I also, after my two bad experiences with predators, don't think they can really be counted on to protect their charges against more than one predator (pack of dogs or coyotes) or a larger predator like a lion or bear. I would get two llamas or three and enjoy them in a herd. They are wonderful animals, fun to watch...