Wat.
When i was a kid i asked people how many legos they had not how many lego they had.
I always used "how much lego do you have?".
I've always used Lego for the singular and the plural.
For me, it works the same way as the words "Sheep" and "Fish".
Both "Sheep" and "Fish" are fine for the singular and plural, but if you say "Sheeps" or "Fishes" then that's also fine, but I'll also think you a bit thick, because it's a short step from there to "Sheepies" and "Fishies".
Edit:
This is a good point.
If I recall correctly lego stands for "Play on" so Legos= "Play ons"
It comes from the Danish phrase "leg godt" meaning "Play Well", so it would be reasonable to say that "Legos" means "Play Wells", which obviously isn't right and doesn't make sense.