One

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...

This is something that always puzzles Americans - but to order a round of toast in the UK you would expect to get just one slice.

I agree with 1

imo a round of toast is 2 because when i say to my mum get me a round of toast she gets me two bits.

 

Once for a laugh i asked her for a toast and raised my glass and she went off and got me one bit of toast. I just looked at her in disbelief.