"Lego" is the plural of "Lego."
I would no more look at a bunch of branded, plastic pieces and call them "Legos" than I would look at slices of bread and call them "breads."
It's the same. It's a slice of bread, a loaf of bread; it's a piece of Lego, a Lego brick a Lego set, a pile of Lego. It's not "a bread" and it's not "a Lego," either.
And just because a bunch of people say it that way doesn't make it any more acceptable. Unless, of course, they all started saying "breads," too.