My clinic would only transfer one unless you were over a certain age and/or had a history of failed transfers.

We only had one healthy embryo (from 2 cycles, 22 eggs, 18 embryos) and that precious embryo is now my 12 month old daughter.

I guess you have to remember that even when you transfer just one embryo through IVF there is a greater risk of still having twins than if you conceive "naturally". Then if you transfer two embryos that risk of twins is even higher. If your clinic gives you the choice of one or two, then you'll have to weigh up the pros and cons of possibly having two babies (or even three or four if one or both embryos split if you have two transferred). We never considered two because I think my husband would have had a heart attack if we had twins, hehe.