You mean forever, even if it dies?
Create a hidden "New Resource". Give 1 "New Resource" to all factions (Using an Ability or an Event). Use that resource as a SpellResourceCost.
Alternatively, use CasterMustBeSov tag, have the spell summon the creature, but also have it UnlockUnitAbility ABC. Use RestrictedAbilityBonusOption ABC as a spell Prereq. Result is your Sovereign can cast the spell once, but once you've done so, the spell is removed from the Spellbook.
Could also try the Tireless March set up, but generally the spell is available again if you dispel it

Sorry, no. I meant only 1 at a time.

I remember the vanilla had something similar, but no matter what I tried, it always allowed me to keep summoning it.

Change <CanStack>0</CanStack> to <CanStack>1</CanStack>
Doable with an item (Style + Color), see: Commander's Wig (WigHelmet_Commanders, Black Market Bazaar MOD).

Thanks!

Tactical Hotbar? Probably possible if you create a new .dxpack using DesktopX from Stardock

Eeesh, that's too much for me XP

Thanks very much anyways!

*edit* Holy quote block mess!!!! I don't know how to undo this!

Tyvm, so would <canstack>0</canstack> work to limit the dragon summons in the strategic map to 1 at a time?

D'oh, 1 at a time per caster :/

Double D'oh, I just found the  canstack tag on almost every spell but not mine. 

Sorry for wasting your time

Thanks again though!

N/A