I don't think you can summon more zombies/skeletons (You get skeletons if you overcharge the spell) if your army is at 20 units or above during a battle. The same as the coming Cygor spell, you can only cast it if your army has less than 20 units.
Addition: I did just check in a custom battle. You cannot summon more zombies/skeletons if your army is at the full size of 20 units. If you have a supporting army you can have up to 40 units but again, you can't summon more units until you have less than 20. If the extra army is A.I. controlled than I would assume you can summon more units if your army numbers drop below 20.
If you start off with 20 units, but lose some during combat, than you can cast the spell.
I do not know if the Cygor and manticore remain in your army after the battle is over. The Cygor may because it's a recruitable unit for the Beastmen (the only faction that can use the Lore of the Wilds) but the manticore isn't a recruitable unit for the Empire or Beastmen so it might not remain.
The summon manticore spell is kind of changed from the tabletop where it's supposed to transform the caster into various monsterous creatures (Manticore, Dragon, Hydra etc) but considering that would require a large amount of modelling and animation for a single spell I can accept the change.