Commander

While questing gets you many followers, I believe that the Inn, achievements and reputation work will prove integral in getting this achievement. WoWHead lists most all of these in their follower guide.

I have a leveling guide that I completed to help alts get their garrison quest chains done and acquire as many followers as possible along the way. Since it is much more detailed, please go there to see how to acquire each of the followers in Shadowmoon, Gorgrond, Talador and Spires. ((Alliance Garrison Quick-Start Quest Guide.)) Below I've listed their names and links to the relevant parts of that guide. At the bottom of the comment, here, you'll also find several non-garrison-related, but easy to acquire followers as well:

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Shadowmoon Valley has three:

• [follower=179.1] - on top of a small rise at (42.9, 40.4).
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• [follower=182.1] - can be acquired upon completion of Game of Thorns
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• [follower=180.1] - is acquired by questing for Fiona at (53.6, 57.2).

More information at: Link to Guide for Shadowmoon Followers

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Gorgrond has four:

• [follower=189] - on top of a plateau, the path up begins at (42.8, 90.9). Challenge him and win.
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• [follower=193] - on top of a small rise at (44.9, 86.9). Defend him then talk to him afterwards.
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• Choosing the Lumber Mill gives: [follower=211.1] - requires a small quest chain that begins with Tangleheart, and [follower=159.1] when quest chain is complete.
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• Whereas Choosing the Sparring Arena gives: [follower=176.1] and [follower=212.1]

More information at: Link to Guide - it won't open to the precise tab needed, though - navigate to Gorgrond's tab and click the link for "Gorgrond Followers and Wrap Up". :-)

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Talador has four:

• [follower=171] - Complete Clear! at (62.9, 50.4). Wait 60 seconds after turning in Clear! and talk to Pleasure-Bot 8000 to acquire it.
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• [follower=207.1] - Complete Gatekeepers of Auchindoun at (57.5, 51.2) just west of Pleasure-Bot 8000.
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• [follower=208] - Complete Dying Wish at (56.8, 26), Ahm should show up in front of your garrison later (at your graveyard?) for you to actually acquire him there.
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• Completing the outpost's quest chain will award either: [follower=155.1] (Artillery) or [follower=154.1] (Arcane).

More information at: Link to Guide - it won't open to the precise tab needed, unfortunately - navigate to Talador's tab and click the link for "Talador Followers and Wrap Up". :-)

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Spires of Arak has three:

• [follower=219] - One of my favorite bodyguard followers. Walk up the ridge located at (54.9, 68.4), beside "Leorajh's Den" and find his cave up top.
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• [follower=192] - part of the Salvage Yard quest chain.
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• Outpost quest chains reward either [follower=453.1] or [follower=168]

More information at: Link to Guide - it won't open to the precise tab needed, unfortunately - navigate to Spires of Arak's tab and click the link for "Spires of Arak Followers and Wrap Up". :-)

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Nagrand has three:
[follower=170] - Climb the tower at (40, 76), take down [npc=80080] and free Goldmane from the nearby cage.

[follower=157] - Acquired via The Honor of a Blademaster after completing the quest chain beginning with They Call Him Lantresor of the Blade.

[follower=209] - Acquired via the quest Abu'gar, which only becomes available after acquiring the following three items: [item=114245] at (38, 49), [item=114242] at (66, 61) and [item=114243] at (85, 39). After acquiring those three objects, visit Abu'gar himself at (67, 56) to turn them in and acquire him as a follower.

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Other readily available followers:
[follower=190] requires visiting 4 separate zones to acquire various items. See my comment here for a concise listing.

[follower=32] is also available to both factions simply by freeing him from two separate cages in Frostfire Ridge: one at (65.9, 60.8), the other at (39.6, 28.0).

[follower=195.1] is acquired alongside a level 1 Lumber Mill.

[follower=194] is acquired alongside a level 3 Lumber Mill and requires a certain number of lumber mill work orders, along with another achievement, Upgrading the Mill.

[follower=455] has to be the easiest of the dungeon-related followers to obtain simply by having the quest and picking up the item from UBRS. He shows up in your Inn at random so it may take a day or two to get the quest, and another day or two to be able to turn it in.

[follower=177] is a bit more difficult to obtain than Millhouse, but not overly so. He is rewarded upon completion of the achievement Come With Me If You Want to Live.

[follower=178] is also available via dungeon achievement Leeeeeeeeeeeeeroy...?.

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Good luck!

Just to confirm, since I haven't seen it stated anywhere else: Inn followers recruited via your Headhunter DO count towards this achievement.

JIC you'd rather skip some of the rep grinds/gold cost for other followers c:

you deactivate followers when you go over your cap of 20 or 25. The cap refers to the amount of active followers you can have at a time.