yes Mercenary Recruitment give you hero's barracks and you can do it 4 times if you don't go craycray Achievement: Master of Alaris will give you another slot and there was one for a preorder so check your claims Finally you can buy slots in the marketplace Keep in mind that slots are just like places to store other mercs. You cannot run more than one merc at a time.
For additional information, I think the quest only unlocks the mercenary slot on the character that completes the quest. The purchased slots from the marketplace unlock the slot for all characters on the account (maybe only the server. I only play on one server).
CatsPaws already gave a compressive answer, but in short . . . . If you want extra slots so you can hire more mercs and switch between them then the answer is yes. If you want extra slots so you can use 2 mercs at the same time, the answer is no.
A 2nd merc would be nice, but I doubt it would ever happen, but I have bigger issues with the 3+ player requirement on various quests. I guess, the merc counts as one. Like being allowed into the planes at level N without doing progression, these lower level quest should just drop the #player requirement at some point. It is just an added hassle nobody needs. Of course, if you are level appropriate it does keep you from pointless death
I just did the Mercenary Recruitment task. While it's easy at level 115. It's nothing but a time consuming slaughter fest. The final piece to complete the task is a rare drop off a named. I found a place where I could cycle through 15 spots where a rare/named mob could spawn. It took about 10 to 12 minutes to cycle through it. I killed close to 1000 mobs and over 40+ named before the Triumvirate Blood Stone dropped. On Sunday I was there about 10 hour and about 3 tonight.
Save your monthly free daybreak bucks and spend them on the additional mercenary slots as your first purchase from the marketplace (unless you want to just throw some actual cash at it to buy the slots sooner). The other options are extremely time consuming to complete, and even then they only apply to the character who completed them. The marketplace slots apply to every character on your account, and the utility they provide is unmatched by anything else the marketplace offers for most players.
Supposed to drop off any named in VOA. I would cycle through the zones killing any named that were up as I ran through. Killed dozens of named and never saw a stone drop so I could finish that quest.
I recently did all the Hunter achievements for VoA. In all the named that I killed (which was practically all of them) I got one stone to drop.
Patients is the key here. I was in Sarith, City of Tides. As I said, there were 15 possible named I was cycling through. I was just killing the PH/Named and any adds that attacked, nothing else. By the time I killed the final PH or named, it was time for another cycle. There wasn't much downtime. I finally got my drop and free merc slot. I was going to stay there until I got my drop. One thing to note. If your into TS's. There a lot of mats that drop that are useful. Plus the mobs drop furniture for your house. I have 3 house (The Wayward Lady) so I got a lot of space for furniture.