In an ideal playing situation I always used 12 characters. In my later years I opted for 18 characters.
Why? Because,
1. I liked having all the buffs available at all times
2. Classes change from time to time, spells, etc so I wanted to make sure I had at least 1 of all classes current.
Classes I never benched were, Warrior, Paladin, Cleric, Shaman, Druid, Mage, Zerker, Monk, Enchanter
If I had to build a single 6 character team....
Warrior, Paladin, Druid, Monk, Zerker and Enchanter
BUT, that's me. This server is so widely diverse. You can literally make nearly every combination work. Some players use SK and Paladins as main tanks, even in the later end tiers.
In the end its a simple formula, Tank, Healer and DPS.
How you get there is subject to how you want to play. Pulling massive amounts of mobs for farming would dictate a different group/raid make up than a progression team in some regards.
Not everyone wants to load 12 characters and a lot of players over the years have had a lot of success with 6 characters only. The fewer the character count the more you will need to progress each character, ie the more side quests to compensate for the lack of raw dps/healing.
Hopefully that gives you some ideas
This is why I have spent time not doing tiers but instead farming what I could sell for soa and mana neck and roa and resistance charms. So that from what I have been told and read when I start doing tiers ill be able to go a tad further before a dps check or ac check hurts me