Once again i find myself agreeing with Girlinhat.
@telamon
Obviously it totally depends on what kind of fort you're building, but a typical breakdown for me would be something like:
1-2 woodworkers, hauling off, doing nothing but cutting trees, making bins, barrels and beds. If they go idle it's because i forgot to designate more trees or order more bins. Only really need 1 but they can die if you're not careful so i like to have a couple - not that i simply let them die though, i protect them obsessively. If they do die, carpentry takes a while to train up.
5-10 stoneworkers, hauling off, doing nothing but mining, smoothing, and building rock furniture. For large scale building i'll turn masonry on for the haulers, and more rarely engraving also for mass smoothing, but otherwise it's all these guys. They are always busy, i like to dig grand (large) halls. I could use more, but i prefer the quality > quantity idea, especially when it comes to mining ores or engraving. Mining gets to legendary pretty fast.
2-20 farmers, hauling off. Usually ~6 farmers is enough for most forts, but once your pop hits around 200 you'll really notice how fast they go through alcohol. Last fort almost starved, i went from 6 farmers to 20, then eventually back to 16 once things were under control. I don't use animals, so i have to grow alot of quarry bushes, which creates jobs for processing the plants, milling nuts, pressing paste, using oil for soap, etc. If they're idle too often, i'll designate the map for plant gathering. If i ever have a surplus of food/drink (over 1000), i'll turn food hauling on.
2-4 craftworkers, hauling off. Leather armor for marksdwarves, and rock pots now and then, but mostly my crafters are making bone bolts. Lots and lots of bone bolts. Again, these guys are always busy (and usually pretty good at bonecrafting), and if there is ever a shortage of bones, i'll make wood bolts or rock crafts if i'm short on
trade supplies goblinite elfinite.
2-6 smelters, hauling off. Depends on the biome and what metals i have, but last fort i had iron/flux/lignite, so i had 6 magma smelters constantly smelting for the entire forts duration. Obviously they were all legendary furnace ops. These guys also cover wood burning, which i need for soap making.
2-4 metalworkers, hauling off unless i'm out of metal - if i have plenty of metal, they might spend most their life making bolts, studding with metal, and making armor and furniture for melting, to train their skills. If i lack metals then i'll have less, and turn on their hauling until i need specific armor/weaponry. Takes a while but i usually get a legendary or 2 eventually.
That covers my main industries. If my idle dwarves ever goes over 5, i check who it is, and if it's one of the above, i give them work to do. Generally they are always busy, and most of them become highly skilled. By the end of the fort, certain jobs start to slow down, like crafting/cooking/brewing, but i figure they've earned the break by then. Because i focus on small groups of highly skilled workers, i have plenty of dwarves leftover for hauling, which is necessary with the amount of produce created.
@nenjin
I totally agree with you. I guess the difference for me is that i'm happy to pick which dwarves to focus on from the crowd of faceless mooks. I'll always know my militia by name, as well as my nobles, and the more skilled of my select few workers - basically, i reward the interesting dwarves with attention. I let the game tell me which dwarves are interesting, rather than stopping at 40 and forcing them all to be interesting. If i get a migrant wave of 30, i don't go "oh god another 30 faceless mooks", i go "oh cool, i wonder if any of them are interesting". This game has an amazing way of creating stories out of the most trivial procedural content, and i find the more dwarves you have, the more opportunity you have for interesting gameplay. Your mileage may vary, of course
Edit: Oh, and i forgot to mention, one of the best parts about having a large population is the fact that migrants are now historical figures. The more dwarves you play with, the more dwarves you modify - ie. the more interesting dwarves you start to create. By the time you're creating your 10th fort from the same civilization, you've probably got a pool of ~500 dwarves that you might actually recognise, which is very awesome. So in a way, having a larger population sort of reduces the number of faceless mooks
Edit again: Legendary bonecarver / artifact creator in the first migrant wave!