I'm thinking of having two. I hope this isn't too many (or too little).

My party is:

Me (sorcerer -- staff)
Jaheira (shield)
Minsc (dual weild)
Keldorn (2handed sword)
Anomen (shield)
Imoen (bow/katana)

I've heard that Jaheira is killer if she dual weilds (and uses Belm in her off hand), but I like her at the back with a sling (or maybe throwing dagger) and as a backup tank. She'll probably use that shield that prevents mind control spells for quite some time.

I always use at least 1 shield-user, usually 2, and rarely as many as 4.

My Tank (Korgan, Mazzy, Jaheira or Anomen) always carries the best Shield I've got, and only breaks out the Bow or the dual-weapons on vary rare occasions.

My Priests always use Shields simply because the selection of Staves isn't worth the loss of AC, and because Jaheira takes forever to be effective at Dual-wielding.

I use one shield, for my (custom) Cleric. You can use Sling and shield at the same time.