There are LEGO tiles, LEGO bricks, LEGO plates, LEGO minifigs, LEGO wheels, LEGO sets, LEGO books etc etc etc. you want to pluralize something, pluralize the item, not the brand.

wrong
if that were the case gooses would be plural for goose and mices would be plural for mouse
It's not even an English word so how can you tack on English grammar rules to it? "Lego", derived from the Danish phrase leg godt [lɑjˀ ˈgʌd], which means "play well". So it's a verb. Verbs are not pluralized.