Conversation so far:
Who won the America's Cup 2017
The challenger, Emirates Team New Zealand, won by a score of 7 to 1 over the defender, Oracle Team USA.
Where was the America's Cup competition held in 2017
It was held on the Great Sound  in Bermuda from June 17 to June 26. the venue was the Royal Naval Dockyard.
What year was the first America's Cup competition
The America's Cup, first offered as the Hundred Guinea Cup on August 20, 1851, by the Royal Yacht Squadron of Great Britain for a race around the Isle of Wight.

Question: How many  vessels compete at a time
Self-contained rewrite: How many vessels compete in the America's Cup at a time