[CannisterFlux]

That should be "it's just grating" ;-)

It's not quite the same as sheep/sheeps. Saying "there are 2 sheep in the field" is fine but saying "there are 2 Lego on the floor" still sounds a bit weird. You would have to use "there are 2 Lego bricks on the floor".

It's because Lego is an uncountable noun, like rice or sand. E.g. "there are 2 grains of rice on the floor" is fine but "there are 2 rice on the floor" is not. Saying "there are 2 rices on the floor" sounds very bad, and is the equivalent to using "Legos" instead of "Lego bricks".