I would have said two, because when you refer to a round of sandwiches you require two slices of bread to make the sandwich. I presume it works the same with toast
I'm not sure people usually refer to a round of toast - it's a term more commonly applied to sandwiches where two slices = one round. When it's toast, people usually refer to the number of slices. Unless it's a toasted sandwich. Mmmm, toasted sandwich, aarrgghh, drool...