Just wanted to make a tread to track this for anyone that cares about it.
This modpack uses 50 of the 55 collision layers.
Total: 50, 24 from Vanilla, 19 from Planets mods (direct), 7 from dependencies (indirect), 1 dupe between dependencies
Optional/recommended mods that add collision-layers (not all tested):
Optional/recommended mods that do not collision-layers (not all tested): Lignumis | Shattered Planet | OMNI | Cosmic Social Distancing | Redrawn Space Connections | Redrawn Space Compatibility | Asteroid Belt | Atomic Arig Lite | Secretas/Frozeta Orbit Patch | Planet Hopper | More Asteroids
EDIT: Removed 3: 2 from Arcanyx and 1 that seems like it was a miscount with Dea Dia.
EDIT2: Added optional/recommended mods.