Is there a way to summon more than 5 creatures at a time. I believe I remember reading something about project image maybe, or simalucrum...

I am a necromancer, so project image is not available. Is there another means of doing this?

Just like you said - both Project Images and Simlacrums ignore the summoning limit. You can literally build armies if you're willing to abuse this.
Dundee has an hilarious picture that demonstrate it quite well. ;)

Unfortunately, your Necromancer can't cast neither of these powerful spells.

In bg 1 I created a ARmY a huge Army made up of Kobolds, Gnolls,Hobogoblins and Goblins. Sent them all straight at sarevok and man he killed most of em and I constantly summoned more monsters with me wand of summmoning but in the end he lost killled by a gnoll