I just bought 5 mercenaries in road of rich, and now, I have a total of 11 mercenaries!
To buy mercenaries, you need the certificate of mercenary expansion, which costs 40 dia. The cost for first, second, third, fourth, fifth, and sixth slots cost 1, 2, 4, 8, 16, and 32 certificate of mercenary expansion respectively.
So, I bought 31 certificate of mercenary expansions to have 5 more slots available! It cost 1240 DIA or approximately 170 Steem.
I highly recommend getting additional slots for the mercenary to get more efficient in mining!