[quote=@Goldeagle1221]
Well, what do you think would be more [I]fun?[/I][@Willy Vereb]
[/quote]Any option aside from the soldiers disappearing after battle or a set time is "fun". Disappearing summoned extra soldiers are convenient and you can use them as meat shields since they are of no consequence. If the soldiers stay then it's going to have amusing social consequences regardless of which mechanism you use. Really the options are limitless and there's no truly best way for them. Just choose whatever mechanism you like the most. Though to make your job easier I give you some examples:

Type A- Clone Army: The simplest mechanism of all, push his army through a magical copy machine. You get two of the same people with the same body, same memory, same skills and same experiences. In case of 50% bonus only copy half of them. So yeah, your army is multiplied in the most literal way possible. This may confuse your soldiers and disrupt order or if you prefer to avoid this (because it'd potentially cripple you instead of providing aid) just add a magic suggestions for the soldiers to ignore this and the leadership receiving knowledge about your extra unit formations gained. Anyways the fun part comes after the battle since suddenly mothers have twin sons returning and wives getting two husbands. There's literally no distinction between the clone and the original. Of course this is going to produce lots of tension and awkward situations as well as some people dueling their magical twin because there can be only one. It's all up to the nation's culture and the individuals, basically up to the roleplayer to describe.Type A1 is when all soldiers get exactly one clone. There's the more chaotic option of A2 when everyone gets a random number of clones. Some get 2 or even 8 while others none. The consequences are even more amusing there.

Type B - Legion Ex Nihilo: Extra soldiers just randomly come to existence. They are no clones but new individuals with their personality and abilities randomized based on what your army is like. In the grand sense the effect is the same as A yet the social issues are totally different. These soldiers are nobodies, born literally by the Volcano's power on the battlefield. Once there's no longer any fighting you are left to deal with thousands if not hundreds of thousands of people who have no relatives or any belongings aside from their battle gear. If you just think about it a little you'd know what kind of utter mess that can bring. Culturally nations closely aligned with the Volcano may even think of them as holy beings or anything along these lines, or the opposite. Seriously, there are so many things you can do. Furthermore unless you make these soldiers sterile (I advise against it) imagine people with the history of being descendants of such people. Hell, if you want these created soldiers may even have unique genes which may be either good or bad for anyone descending from them.

Type C - Retcon Army: Last big category is just as amusing. What if the boon of doubling your army has retroactive consequences? These new soldiers are like Type B but have relations like Type A. Except these relations and belongings are all fabrications which didn't exist before the boon. You don't just get extra soldiers but their relatives, economy and all that is necessary to support them. It's kind of convenient but while reality changes the memory of people mostly remain the same. It's a big country-wide mess which would give plenty of roleplaying options, as well.

So yeah, there are so many amusing options to choose from. Although on a second thought perhaps you don't even need to choose here at all. If you don't mind another roll just add all of these options and let the dice pick one at random. That way things can get the most screwy.

Speaking of which I hope you have nothing against our nations' history involving the effects of boons and curses that once hit them. For example being the son of an "Ex Nihilo" noble or a "clone" prince. Hell imagine a country which has a history of once suddenly gaining 8 kings. It's just too amuding to miss, IMO.