In my single-player, Hard campaign game – I've found that Metsuke (the secret police of Shogun 2) agents are useful to boost income and catch enemy ninja. What confuses me is why I can't hire any more – the tool-tip indicates that it is based on the number of market-chain buildings I have, and I am sure I have more markets than Metsuke.
Also, it is not clear whether an upgrade to a market counts as an extra 'market-chain' building – I am guessing that it doesn't.
Best Answer
Asked 2 weeks ago and if no-one has further information will accept this as the answer:
From browsing forums it seem there is a hard-limit of 5 agents of any type, regardless of how many chains you have. You can never have more than 5 Metsuke, 5 Ninjas, 5 Monks, etc. http://forums.totalwar.org/vb/showthread.php?134068-Where-to-build-markets-only-in-fertile-lands/page2