Determinging sprite assembly

How does the game determine which sprite assembly information to apply to each sprite?  C0CE3A contains the patterns for the actually assembly, but I can't figure out how it knows which one to apply.

For instance, sprites 0 through 21 use the first 516 bytes from that area then switches to a different section for sprites 22-25, then switches again.
There doesn't appear to be a table that it's looking up.
I've gone over the code that seems to load the images C06777, but I can't figure out how it determines when to stop.  Best I can tell, it's based off decrementing the memory at $18, but it looks like that's just set to 6 at C067A5.