In "Wii Play" how many tanks do you need to destroy in Tanks! to earn a gold medal?

Eighty-four tanks.

160 tanks-platinum
84 tanks-gold
40 tanks-silver
20 tanks-bronze

Only 20 missions are initially available for play, but 80 more (For a grand total of 100) missions and various new enemy tanks are unlocked once the player has earned a gold medal by completing all 20 missions. In multiplayer mode, only the first 20 levels are available.

http://download.cheatcodes.com/faqs/67948.html
https://en.wikipedia.org/wiki/Wii_Play