The earliest reference to a 'round' of toast appears in Barnaby Rudge, published in 1840, by Charles Dickens. There, the sentence reads: "A couple of rounds of buttered toast."
The dictionary says a round may be defined as a) 'a piece of bread cut right across the loaf' or b) 'a sandwich made from two slices cut from a loaf'.
I'd say, therefore, that ' round' in terms of toast is one slice and that 'round' in terms of a sandwich involves two slices.
If you're ordering a round of toast in a caf� I suppose it is whatever one serving of toast consists of. i.e. if a caf� serves a portion of toast as two slices then a couple of rounds of toast would equate to four slices.