There are 24 Anarky Tags in Gotham, but you only need 20 for this trophy. To scan them, you will need to stand in front of them or near them and hold down . This can be a tough hunt for 2 reasons: 1, they are only visible in Detective Vision (they are orange), and 2, you can't check which Tag you already scanned and where they are.

It's better if you use this map below as a guide for where to find the tags, and check the ones you already have. That way you'll avoid scanning the same tag multiple times.