R.X parameters for decal break Shader compilation

Hi,
Can someone explain me why adding those parameters break the Shader Compilation of the ShooterGame Weapon Rifle?

[SystemSettings]
r.EarlyZPassMovable=1
r.EarlyZPass=2
r.DBuffer=1

Errors: [SM5] Errror x4510: maximum ps_5_0 sampler register index (16)

Those settings are set by default in UT, and seems to fix issue with Decals.
I just need to understand why it affects the weapon material so I can learn and prevent future issue with my final game assets

Thanks

Can someone help me to unserstand this?