Don't have Rome 2 yet but from what I've seen the slots are really limited forcing you to make a ton of compromises. Which can be quite weird when your Rome doesn't even have half of what it has in real life. I'd just wait for patches since as of now the game isn't worth playing except for laughing at the funny glitches to be honest. A province has a flat 3 recruitment slot, constructing additional unit producing structures will not increase the number of units that can be recruited per turn. However additional recruitment slots could be achieved through the use of edicts and techs.
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