Okay, so samplers don't count unless the compiled code actually uses them. Having said that, Unity 5 can potentially use as many as 7 samplers if the project happens to be using specular directional static & dynamic lightmaps while blending between two reflection probes. Though this is a pretty unlikely scenario, as most people will pick either static or dynamic lightmaps.
So the culprit is in the auto-generated "PrePassFinal" pass for all surface shaders. Here we have a Light Accumulation Buffer plus two lightmaps (either Dual or Directional). Apparently it can go as high as four textures on the Xbox 360 in HDR mode. :(
Unity reserves 3 samplers for its own use in its surface shaders system. These transparently allow all surface shaders to support things like lightmaps, light cookies, shadowmaps, etc. This leaves exactly 13 samplers for the shader author to use for whatever they want in their custom surface shaders.
For the vast majority of people, this is a non-issue. After all, when would a shader ever need 13 samplers? You'd have to be doing some really complex effects-heavy shaders with custom IBL...like the ones we are including in Alloy.
Once you have something like "Illumin/Detail Bumped MaskedIncan Rim Dissolve", you hit that limit pretty quickly. Then you end up in the situation I'm in where you have to decide where you can squeeze out or just plain drop a texture in order to free that sampler up for use elsewhere. So it's better not to waste samplers by planning out EXACTLY how many you will need beforehand.