Basics of the team.

Thief: All round character. Tends to be the swiss army knife for everything except mobs that dont like thieves or circle. Everything else tends to go to one of the below classes. You should get a full complement just because. Even then, 8 can turn out not to be enough if you need backups and whatnot.
Warrior/Paladin/Ranger: If it isnt a thief run, chances are you use a nasr warrior. Also prefered by alot of people to tank on these as all you do is quaff.. nasr does the damage. You just watch the screen scroll and hit quaff every so often so you dont die. You want at least 2 Dev. maybe 3 depending on what your goal is to run. But 3 is all you should ever need.
Fathomer: The new thief basically. Vindur gives them an offensive advantage (though its rather costly as a) you need to start quaffing manas too which means you spend more time quaffing b) manas cost money. c) they have less hp than a thief (probably) which means you need to quaff more often for lesser amounts d) you need to learn how to get a GOOD ratio so you use heals and manas at a proper ratio which depends entirely on the mob. but running out of either isn't fun, and you STILL only have 500 weight to pack it all in.
On the other hand, the fathomer packs 2 attack skills which compare with circle, both are decent in their own right but both have their limits so it is probably harder to run on a fathomer than a thief. 1 is a good start for running. 2 for when you need backups brings the total to 3... when you get there.
Barbarian: All star hitter really. rend got upped % wise so its now a very decent skill... other than the fact you lose parry altogether, the melee as well as the saves and hp/whatever else that counts on weapons. But it definitely makes for an interesting runner especially when you just aved and dont have gold for weapons... Sweetest thing is you dont NEED weapons for this guy most of the time. Charge is nice too. but anything that involves a tunnel and its worthless. Rend outperforms charge anyways. Now just give me a pacifist mount so stupid mobs can stop killing my mount. 1 to hit, then aim for a second once the rest of the team is done.

Support classes: These tend to be the less important classes. They really can't do the killing unless there are special circumstances.
Mage: Balance, defense, quill. Thats the order you should get em. Its common sense really. Everything else is basically worthless btw. Counter is very limited (and basically to only ONE mob. though its good if you kill that one mob alot) Paradox is outshined by vindur and rend. forget the destruction mages people. They sound good, are crap. You need to take into account the fact mages only have 350 weight and running out of potions is a less than satisfying way to die. Quill mages = awesome scribers/brewers. shadowform takes 400 or so mana to brew. these guys can churn em out. 2,2,2 is what I go by. I'm actually working on a 3rd balance so I have a second backup.
Cleric: You should know why this guy is important. Slightly less so than it used to be because bought heals are cheap enough to not warrant brewing of heals. BUT you still need to brew sancs, cast holy/minor/bene/indig. One tends to be enough till you really have more than a team's worth of characters.. then you need 2-3. Anymore is overkill.
Nephandi: Alot of unique spelldowns and makes gremlins for realigning. Keep just one around as it won't see an awful lot of use with qlip being more costly than vindur (deals less, more lag and eats pentagrams too)
Druid: Kinda an all round character but not excellent for anything. High rescue is nice and has decent elemental defenses. 1 should be all you ever need.