You can vary the people you use in each stage of the lovingkindness meditation. You might want to call to mind the same person in the friend stage for a while, but it’s good to work your way round all of your friends.
I find that I prefer to consistently cultivate metta for one particular friend for a while, by which I means weeks or months, because I get a sense that I’m really working on strengthening the caring connection that I have with them. But in the long terms I wouldn’t want to stick exclusively with one person because, obviously, I have more than one friend and want to deepen my connection with each of them.
The situation’s a bit different if you’re new to the practice, of course. You may have a half-dozen or more people that you could call to mind in the second stage of the meditation practice, but spending months on each of them would mean putting off for months or years cultivating metta for those that were at the end of your list! So I’d suggest perhaps spending a few days or at most a week or two on each person.
When it comes to the other stages — for example when we’re developing metta for someone we don’t get on with — you might find that you have to stop putting a particular person in that stage because you no longer have any ill-will towards them!
Watch out for restlessness — wanting to constantly change things to keep it interesting. That can be a hindrance in itself. If you’re cultivating metta for a different friend every time you do the practice you may not really develop much momentum in the practice. So it’s fine to work on the same person for days, or weeks, or even months. If you do use the same person for a while, you’re also more likely to notice the changes in your attitudes towards them as your practice begins to have its effects.
Again, it’s a very good idea to decide before the meditation begins which friend you’re going to choose. In fact it’s not a bad idea to have a “default” friend so that if there’s any doubt you just call him or her to mind right away. Indecision can be a hindrance in meditation, and you don’t want to be stuck wondering who to send your lovingkindness to.
If you do forget to choose a friend before you start meditating, don’t waste time mulling over which person you’ll call to mind. Just pick someone! As long as you’re dithering you’re not meditating!
It’s also deeply unpleasant to be full of doubt, wondering if you’re being unfair to one friend by calling to mind another. Just do it! This in fact is an interesting indicator of how messed up our emotional lives can be: when we’re doing something that’s undoubtedly a very ethical and positive action we end up undermining ourselves by constantly wondering whether we’re in fact doing it imperfectly. But these kinks will work their way out of the practice as we continue to do it. As we become more emotionally positive we start to recognize how futile and draining this kind of second-guessing is. It becomes more important to cultivate metta than to worry about whether we’re cultivating metta “properly.”