Ten Tabards

According to Tigole as soon as the Achievement counts the tabard you can delete it or sell it back to the vendor.

The Trophy Trophy Tabards of the Illidari, obtained from the Crimson Watch quest line can be used to get two Tabards closer to this achievement, and the 25 version.

Complete the following quest, and it's follow-up http://www.wowhead.com/?quest=10793

You will then obtain one of two tabards from the reward chest.

http://www.wowhead.com/?item=31404
or
http://www.wowhead.com/?item=31405

Now, you can not redo the quest, and can never have both tabards at the same time. But once you have one it has counted towards this achievement, and the 25 version. Now delete the tabard you have, whether it green or purple. You can now visit a tabard vendor in any major city and they will have an option for you to collect from them either the green or purple tabard, simply ask them for the one you didn't get from the quest. Free point towards the achievement.

you can re-acquire lost awarded tabards at the tabard vendor, but the tabard vendor also sells all reputation tabards that are available to you