Who was the first person known to sail completely sail around the world?

Magellan

It wasn't Magellan. He was killed in the Phillipines. It was actually members of his crew that made it around.

And I think as few as 18 who left in the first place actually made the entire voyage.

I'm pretty sure that Ferdinand Magellan made the first attemp but was later killed, but his crew was the first ever to sail around the world in 1519. He set sail from Spain, then crosses over the Atlantic, then rounds South America's Cape Horn, crosses the Pacific Ocean, arrives in the Phillipines. That's when he is then killed. His crew then makes it back to Spain in 1522.