I have a group which is 1x Prot Paladin and 4x Shaman.
95% of the time, it is 4x Elemental, although one can dual-spec to Resto.

Every key is in the same place for me Elemental shammies.
I have a single click macro for their dps.
In addition to this I have: Totem Default, Totem AoE, Totem PvP, 4x Cleansing, 4x Grounding, 4x Magma, 1x Mana + 3x Healing, 4x Searing.
I also have Earthbind, Fire Elementals, Fire Nova, Thunderstorm and Tremor on round robin.
That's a few keys.

My Paladin has a click sequence.
A few keys for buffs.
I have Avenger Shield, Divine Plea, Hammer of Justice and Blessing of Protection as unique clicks which I don't want to be part of macro's.

If I were to add another class, say drop a shaman and add a Disc Priest.
I would need keys for Mass Dispel, Power Infusion, Pain Suppression etc.

The more things I have to click, the more complicated the set up becomes.
The fewer classes, the less things you need to remember.

A group of 5x the same class is easier to set up, and fewer things to click on in play.
Then a group of five different classes.
A lot of stuff will overlap, for example spam dps and spam tank keys.
But every class has some things which you want to use independently.
And the more independent features you include in your set up, the harder it becomes to play a team.