From a roleplay perspective, the ammount of classes any given game has is effectively "as many as you want". Just as there are no scientist classes, there's no masonry, architects, businessmen or thieves in the mechanics of the game. But that doesn't mean that there is no actual in-universe people spending their lives doing those things. In fact, it's pretty unreal to think those don't exist, unless the lore states otherwise. Using that logic you can infer that there are likely masons, architects, businessmen and dagger wielding thieves in the world. The same way that not all people using swords and shields studied in the Gladiator's Guild, or that not all spear users come from Gridania, can be used to justify the existance of other 'classes'. It's just that the game doesn't represent them because it would be pretty inconvenient: the same reason cities aren't populated with tons of NPCs or why you can travel all across Eorzea by foot in a few in-game days instead. It's scaled down for convinience.
The problems come with classes/jobs that can't be properly infered from the current setting. For example, currently ARR (as far as I know, anyway) doesn't have a good way to justify samurais or ninjas. But there are ways to justify thieves and musketeers: we have daggers, there's no conceptual barrier stopping characters from stealing things or being stealthy; and there's gunpowder and firearms. The only thing you can't do is say that there are actual guilds for those, but you can create a semi-canonical lore by simply creating the backstory. After all, and as I said before, there's no reason to think that all swordmen studied in the Gladiator's Guild. So there's no reason to think that thieves, musketeers or whatever had to come from an actual guild. All you need is the proper weapon and thematic skills.
The problems come with classes/jobs that can't be properly infered from the current setting. For example, currently ARR (as far as I know, anyway) doesn't have a good way to justify samurais or ninjas. But there are ways to justify thieves and musketeers: we have daggers, there's no conceptual barrier stopping characters from stealing things or being stealthy; and there's gunpowder and firearms. The only thing you can't do is say that there are actual guilds for those, but you can create a semi-canonical lore by simply creating the backstory. After all, and as I said before, there's no reason to think that all swordmen studied in the Gladiator's Guild. So there's no reason to think that thieves, musketeers or whatever had to come from an actual guild. All you need is the proper weapon and thematic skills.