Hm... I guess you're right about chapter 24. There's a line about how they got there and are around due to Grimma's power, but that doesn't necessarily mean that more will appear during the battle. Anyway, they still only appear from points that are basically marked on the map though. Even in maps without fortresses and/or stairs, you can easily have a general idea of where they'll come from too.
Take the boat chapter. Like I said, even in my first playthrough, it was pretty easy to figure out that the reinforcements could only come from the edges of the map, so I didn't take my units close to it and stayed in the middle until they stopped for several turns. On the other hand, I didn't want to deal with reinforcements in the tree chapter, so I finished it as quickly as I could, following the story plan, only letting one wave arrive. If you take too long and get surrounded there, you failed to execute the strategy that the characters talk about before the chapter and is being punished for it.
In this game, at least, only in Lunatic and above you really need to make bets like that. It's perfectly possible to go through hard with close to 100% efficiency with many different party configurations. If you want to avoid hit chances, have some weaker weapons with high hit rates on hand. If you want to avoid low critic chances, attack from rangers where you can't be counterattack, at least by the weapon with the crit chance, or just attack with a unit that can survive even a critical hit.
This is actually why I kind of dislike doing speed runs or worrying about speed rankings in Fire Emblem, since it forces you to take chances, sometimes even reseting upon bad level ups, depending on the game.