At launch, there were only 12 maps in the game. Going off the achievement description, I'm assuming they have it set up for whenever they add new maps, but we'll have to see if that works once they add them. They've stated any future maps will be free, so if they coded the achievement as it's worded it should become easier as time goes on.
The original maps are
- Dorado
- Hanamura
- Hollywood
- Ilios
- King's Row
- Lijiang Tower
- Nepal
- Numbani
- Route 66
- Temple of Anubis
- Volskaya Industries
- Watchpoint: Gibraltor
Later Maps - Should Count
- Eichenwalde
- Oasis
- Horizon Lunar Colony
- Junkertown
- Blizzard World
- Rialto
- Busan
- Paris
- Havana
Since there's no way to really search for games your only option to actually try to go for it would be to keep quitting if a map you already did came up. Matches are pretty quick in this game though so it'll come with time. It's all luck of the draw.
On every game mode except Control (Ilios, Lijiang, Nepal, Oasis, Busan), you play the map as both sides. In Quickplay, each of these is a separate game, so you have 2 chances in a row to win on that map if you need it (assuming you didn't join in progress on the second half).
As far as I can tell, there's no in-game tracker for this, so you just need to keep track of it yourself. Also, the achievement doesn't seem to track how many you've done with the percentage, but that wouldn't help you anyway if you didn't know which ones you've won on.