This topic has been discussed a lot. You'll find many opinions on the topic if you search. I personally xfered 2, but my situation is unique. I'm using a proven surrogate and have had multiple failures and cycles. I'm not trying for twins, but I wanted to increase my odds for one because of the expense and complications that come with a surrogate. My surrogate is young, proven, and has an excellent chance of having a very healthy twin pregnancy. If I was able to carry my own baby, I would transfer only one.
Many clinics are advocating only one and are openly discussing the risks associated with even a twin pregnancy. Of course there have been many healthy full-term twin births, even here on this board, but it is a high risk pregnancy. Also, if one's infertility is due to her uterine environment, rather than embryo quality, then it's possible that no embryos will develop, no matter how many are transferred.
If you are looking for advice, I would suggest that you do lots of research and understand all the risks involved. Then make your decision.
Here's a good article from Shady Grove Fertility.
Editing to add this one as well: