You know, you can only summon one Deva or Planetar at a time. Does anybody know where or which file defines this limit?

It's in the spell file, itself, if I remember correctly. If you wanted to, you could remove the limit with an editor like DLTCEP or Near Infinity.

A projected image will be able to summon multiple planetars without changing anything in the game.

A projected image will be able to summon multiple planetars without changing anything in the game. You misunderstood me. I want to change the Summon Deva spell so that I can summon a Solar when my caster level reaches lvl30. But I want to limit it to only one.

Oh, you could use the Deva as a template. Just copy the Deva .CRE and .SPL files and edit them to use the resources you're planning the solar to have (sounds, BAMs etc). Then its just a matter of tweaking the memorised spells, hitpoints, etc. and voila a Solar that should thoretically have the same in game restrictions of the deva.

I'd say that it would be an obscure flag somewhere on the .SPL file.

Take a look at any spell that has multiple level effects in Near Infinity or DLTCEP (Fireball and Dispel Magic are both examples of spells who's effects are level dependant). Use that as your template and making your spell conjure a Solar at level 30 is a snap. Don't worry about the summoning limit. Just adding a minimmum level 30 effect to summon a Solar to the spell file will get the job done. To make a Solar, I would reccomend just altering the stats, spells, etc. of the Deva.cre file and renaming it. Don't use the Solar.cre from the game. The item affecting summoning limit might actually be in the .cre. If you just alter the stats and whatnot, this won't be a problem.