Hey Caladrius, don’t know if this is a certain fix for you, but it may be worth looking into. I ran into a similar problem a while ago and got some good guidance on how to fix it.
For me, it wasn’t a matter of how many enemies there were, but rather, how many different classes were on the map (ally, enemy, player unit… all of them together). Every map sprite uses some of the emulator’s OAM, and if you exceed it, you can start to have funky stuff happen.
I’m not sure if you have NO$GBA, but it would be worth downloading it to see. When you load up your rom and get into whatever chapter is giving you problems, go to the Window tab and select OAM viewer:
From there, go over to the OBJ tab. Here, it will give you a rough graphic of the map sprites currently being used in the chapter.
What I found for myself is that the 8-16th rows are used for map sprites. If you have too many different map sprites in a chapter, you will run out of room in the OAM and strange graphical issues will start to happen (usually due to overlapping sprites). My guess is this is what is happening to you right now.
I’m sure someone more knowledgeable could add something more here, but for me, there were two fixes for this problem. The first was to just simply have less class diversity in the chapter. The second, while requiring slightly more work, is a better long term solution.
Some map sprites, both in the repository and even those in vanilla, use more OAM than they really need to. You can see this by loading the map sprite into some graphics software (I used Microsoft Paint). If you look at the OBJ viewer above, every square represents and 8x8 square of pixels. Most sprites that were efficiently made only need 16x16 pixels, some of them are larger vertically to accommodate larger sprites, but what you’ll see quickly in this viewer is that some sprites are too large for an unnecessary reason. Some vanilla sprites even use 32x32 pixels when they can easily fit into 16x16.
The fix is to edit the map sprites to fit into either 16x16 sections or 16x32, which means some minor adjustments to where the map sprite is placed (which I also did in Paint). Try to avoid having them any wider than 16 pixels… Many of the larger sprites are given extra space vertically instead (you can even see it above with the cavalry).
Hopefully this helps.