How much of the story will I miss if I don't take any NPCs into my party?

Quite a bit, actually.

You would sure miss a lot of quests without the NPC's, particularly Nalia. After all, Nalia's keep quest is your first way out of the city, plus gets you a lot of gold and goodies. I understand not wanting very many NPC's (EPs get split up fewer ways, fewer compications, easier to spread spells and magic items around), but NONE ?
Personally, I like to run with 4 characters ( me, Minsc, Nalia and Aerie) adding Imoen when I rescue her. Also, I brought in Anomen to help w/ Chapter 6, but sent him home when I was done. The poor fellow was a bit cheesed off by this, but he was a bit wimpy at that point- I just needed someone to help defend against undead.

Personally I think you should pick up ALL the NPC's at some stage. Complete their quest then dump em. Yoshimo's is awesome, Edwins is good, Mazzy's is different....
There is much more to this game than simply completing it.

u'd miss these quests

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planar prison (aerie)
planar sphere (valygar)
anomens murdered sister (anomen)
anomens knighting (anomen)
nalia's father's funeral (nalia)
nalia's kidnap (nalia)
yoshimo's heart (yoshimo, murder him in assylum?)
baron ployers curse (jaheira)
umar hills (minsc, although u can get it from the cowled wizards)
and there r loads others, just none spring to mind at the moment. in the above quests ur talking about well over 100,000 exp , no probs.

Spoilers
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Planar Prison? You mean the one you get in the bridge district? You don't need Aerie in the party to do that, it's just Quayle won't tell you about it if you don't have Aerie in your party. Umar Hills you can get from Delon, who you can find in the Government District if you don't have Minsc in your party.
Some of the ones you would miss in addition to the others listed:
Korgan's Book
Cernd's Child
Jaheira's Harper Quests
Keldorn's Wife
Jan's Family

About the planar sphere - You can kill Valygar and take his body there, rather than actually having him in your party.

I'd say well over 1,000,000.

Also....

Mazzy's Duel
Edwin and his scrolls
Jaheira's "ultimate" quest, you must be romancing her.
Korgans
the list grows.

Incidentally, NPC's (whilst in your party) can interract with each other which can add new elements to the game.

have you played through before? i'm assume you have, if so how many times?, many people do solo but they have pretty intimate knowlegde of the game

I played through twice and learnt loads. I then solo'd a multiclass fighter/thief. The replayability factor of this game definitely makes it one of the greatest games I've ever played.

You can't play the game "wrong" so don't worry there. You just learn and develop the more you play it. For me the second game was better than the first (although I enjoyed the first immensely) and the solo the best of all.