Beware of laying too many farmers too early on farms that are quite close to each other, lest cunning opponents join up all your farms so you end up with one farm, which may be quite large, but only scores you the points for the cities once, not for the multiple farmers.
AND beware of farm 'thieves' - my friend is a master of this - he'll wait until the last 20 or so tiles, then start adding farmers on tiny bits of what look like non-point-worthy sections of field, and then on subsequent moves join them up to MY farms - usually with a 2-point meeple (from Inns & Cathedrals expansion) or actually two or more meeple, thus depriving me of my huge farm score I'd been building up the whole game. The swine! :)
EDIT: I released I didn't actually answer the question posed. I usually use around 3 meeple for farmers during a game, then try deploying any and all remaining meeple as farmers near game end to grab last minute points from any remaining un-farmed fields. Deploying too many farmers too early on can lead to the above noted things happening, by which time you have little way of gaining any foothold back!