User blog comment:IamJakuhoRaikoben/Chapter 439: Albareth Empire, Review./@comment-5969537-20150610014537/@comment-7418318-20150610063052

It isn't just about raw numbers. Those 500 guilds were mentioned to give the characters and the readers an idea of the numbers, but that does not mean those 500 Ishgar guilds would really all meet the invasion force in battle.

The Alvarez Empire has 730 guilds within a single country, under a single command structure. If it comes to war ALL of those 730 guilds will fight united for a single goal without question.

The entire continent of Ishgar together has 500 guilds, spread over at least half a dozen countries. We don't know if those countries are allies, neutral or even hostile to each other. For all we know there could be a war between Bosco and Seven right now and we just didn't know it, because it had no impact to the story and thus wans't mentioned... or a cold war between Ca-Elum and Fiore.

If Alvarez attacks only Fiore, Fiore might well stay alone and from the GMG we know they have less than 117 guilds, or at least had back in X791.

Even if all those countries would send reinforcements, those 500 guilds would still be loyal to their own country first and the continent second. Though I seriously doubt any country would send their full complement of guild mages into the fray so we're more likely talking about 400 to 450ish guilds tops.

With the other countries not even knowing of Lumen Histoire's existence, the enemies plans are a mystery to them. Since they don't know the target they are aiming for is in Fiore, they'll understandably be concerned about their own countries being invaded and thus will keep appropriately strong defenses back there.

Another problem is that, as someone else commented on further down, Fiore's guilds are generalists with abilities and personell suited for a wide variety of tasks. From this discription we got Alvarez seems quite militant which likely means their guilds have a higher number of pure combat specialists... again a point in favour of the invaders.