Build buildings that increase the army garrison. People love using mercenaries as a emergency relief if they got the money to afford the upkeep.

that paradox published title which is exactly the main reason i dont put it them in high regard. outside magicka, warband and sots (te 1st) their published game are meh some of them i agreed have something diffrent is just that the bug and following up content most of them are lacking. their own development game before ck2 are also high advissable to avoid like a year or so. sure they promise their qa quality after sots2 will put more on more priorities but still reputation is something...
as for ck2, highly advisable watching some play or just like me ready to open up http://www.ckiiwiki.com/, it is actually easy to grasp. to increase your count simply click on the city you own and controlo not your vasall btw is also depend on your martial stat ;p and like cw8 say prepare for some gold just in case you need the extra merc