When you're about to battle in this game almost always you are able to use ADD option to increase the amount of monsters you want to have in fight.
Some "Elite" monsters can only be added once for each battle however. And i said some, because other "Elite" monsters can be added up to maximum amount if wanted.
Why are certain "Elite" monsters so special that there can only be 1 of them for each battle?
Best Answer
I don't think this applies to all situations, but for some monster types such as (very mild spoiler):
their limiting factor is size - there is simply not enough room for more than one of them at a time.
My guess as to why you can't add others is that this makes it harder to farm elites who are likely to drop treasure, so as not to flood the players with gold and make it a little more likely that players will purchase it instead. This last part is speculation on my part, though.