I'm not quite sure about English, but in Dutch, for single lego pieces we use the equivalent of "lego piece", or just "piece" when the context is clear, which p

I'm not quite sure about English, but in Dutch, for single lego pieces we use the equivalent of "lego piece", or just "piece" when the context is clear, which pluralizes normally. If you say "I'm going to play with lego", the word "lego" is used in a more general, abstract sense, which has no plural at all.