"A pride of lions", "a pack of wolves" are examples of collective noun.

Lego does not fit that pattern.

Sugar, Bread, or water are example of uncountable nouns.

"A gram of sugar", "a loaf of bread", "a litre of water", "a bag of Lego".

You also wouldn't say "one lego"; you might say "one Lego piece" or "one Lego 2x2 piece".