i know you probaly have heard this q hundreds of times befor but anyway
what kind of specialst mage should i make for my multiplayer party
and what kind of classes should i use,how many guys should i have in the party

Get 2 mages, specialists that are opposites, like Diviner and Conjurer, Transmuter, Abjurer. So then you can get all the spells.

I'd recommend a Conjurer. I'd also suggest you get a backup spellcaster, either another pure mage or a dual/multi-classed one. or a sorcerer.

Conjurer. ID glasses get rid of your identification problem, and your planetar will handle true sight for you.

play a sorceror, you wont be let down. as far as specialist mages, i go with either necromancer, or invoker. that is just me though.

In my opinion, Sorcerers really do leave mages standing because of the sheer amount of spells they can use before needing to rest. However, if you still want a greater variation of spells that a sorcerer cannot achieve, then Cleric/Mage is another wicked magic user.
~ThERaGe~

Is everyone here overlooking the Wild Mage? They prod buttocks! Besides having the extra spell per level of specialist magi without any restrictions to spell schools, Nahal's Reckless Dweomer gives them access to high level spells at low character level. Agreed, there is a small chance that the result isn't what you hoped for, but that's a small price to pay, IMO.

As for your other question, what classes to use, that really depends on your playing style. What usually works well though, is to get two of each except for thief. So two warriors, two wizards, two priests and one thief. My current multiplayer party for example consists of a Wild Mage, a multiclass Thief/Mage, a Berserker, a Cavalier, a Priest of Lathander and an Avenger.
If you're very centered on melee however, you can exchange on of the spellcasters for a warrior, or vice versa.
Hope this helps ya.