My opinion is at the middle. I think it is good to utilize the merchants-clerics (not the spies) ,by giving them a limited amount of units, just for the province security, such as rebelion or enemy assaults, while your marshals will be on an expansive mission. The unit capacity for example can be 4 groups of certain units (for instance the cleric or merchant cannot command feudal knights or cataprachts). That's because merhants are useless(for me) in late game, cause the income that produce is too little compare to the tax income from the provinces and i prefer to use my royal slots to have many marshals to control better my "huge" empire.
My thought over how many groups can a royal court member can have is:
Spies: 0 slots of units (as he works alone only)
Merchants-Clerics: 3 or 4 slots (for inside-the-province missions)(only basic unit usage such as bowmen-archers, militia-swordsman and light cavalry)
Marshals:6 slots (as in the original game)
Princes: 7-8 slots (as bonus cause he is from the royal family)
King 9 slots (you want to follow your own king anywere, do you?)