Math / Maths. Legos / Lego. There’s other examples too where Bri’ish and Septic English differ on pluralisation. It’s what happens when language mutates.
According to LEGO the term "LEGO" is an adjective and therefore never plural (LEGO bricks). That being said you can't control how language develops, so lego/legos it doesn't matter as long as you're understood.