I rather think that this is a way that Darkpaw hopes to get rid of DB coins that people have gathered up and not spent them on anything yet.
They sell a 30 pack (each containing 3 agents) + 3 bonus, add in the 10 original. If there is a cap, its at least that many, though I might be surprised if there is.
The bottom 3 quest will take your guys away and trade them for others. Don't do those quest and you'll have plenty of guys.
It's 2020, people don't know how to read anymore. They just want to be told on what to click on. So yeah...don't click on the bottom three quests. Tell your friends!
Make sure you do the recruitment quests, its 48 hours and reward is more agents. Also if your really needing more spend some of the free dbc you get on a agent pack. Not really necessary though. You also do not have to fill every spot in a quest.
I have not gotten any recruitment quests in my list yet. I have done a lot of others but still hoping a recruitment one will show up.
There's a bit of luck involved in this as the recruitment missions require soldiers. Only 1 out of the 6 characters I'm running Overseer on have soldiers -- so you'll most likely have to buy a pack or cross your fingers on conversion.