I don’t refer to brand names much, but I can state a rule: Do it the way the owner of the brand wants. Take a look at this:
The Lego company says that “Lego” has no plural. If you want to refer to one of the pieces, call them “Lego blocks” or “Lego pieces.” But don’t call them “Legos.”
So—It’s a box of Lego, not a box of Legos.