Lego exist only in plural. Something one is not Lego. Like sand. I think.
Technically, Lego is an adjective as it's derived from the Danish words for 'play well' so sticking an 's' on the end doesn't make any sense. If you read any LEGO literature or web page you will never see the word Legos. In fact they have gone further as the American insistence in pluralising their name clearly annoys them. This is a quote that used to be on their web site:
“The word LEGO is a brand name, and is very special to all of us in the LEGO Group Companies. We would sincerely like your help in keeping it special. Please always refer to our products as “LEGO bricks or toys” and not “LEGOS.” By doing so, you will be helping to protect and preserve a brand of which we are very proud, and that stands for quality the world over. Thank you!”