Shader with 16 sampler2D variables exceeds sampler limit of 16?

I am working on a custom shader inspired by this post.

In the shader, I am using 16 sampler2D variables, but Unity keeps telling me "sampler limit exceeded; more than 16 samplers needed to compile program" Am I doing something wrong, or is this a bug? If a bug, is there a known workaround?

Thanks, Florian

 Shader "Custom/WorldShader16" {
 Properties {
 _MainTex1 ("1", 2D) = "white" {}
 _MainTex2 ("2", 2D) = "white" {}
 _MainTex3 ("3", 2D) = "white" {}
 _MainTex4 ("4", 2D) = "white" {}
 _MainTex5 ("5", 2D) = "white" {}
 _MainTex6 ("6", 2D) = "white" {}
 _MainTex7 ("7", 2D) = "white" {}
 _MainTex8 ("8", 2D) = "white" {}
 _MainTex9 ("9", 2D) = "white" {}
 _MainTex10 ("10", 2D) = "white" {}
 _MainTex11 ("11", 2D) = "white" {}
 _MainTex12 ("12", 2D) = "white" {}
 _MainTex13 ("13", 2D) = "white" {}
 _MainTex14 ("14", 2D) = "white" {}
 _MainTex15 ("15", 2D) = "white" {}
 _MainTex16 ("16", 2D) = "white" {}
 
 }
 SubShader {
 Tags { "RenderType"="Opaque" }
 LOD 100
 
 CGPROGRAM
 #pragma target 3.0
 #pragma surface surf Lambert
 struct Input {
 float2 uv_MainTex1;
 };
 
 sampler2D _MainTex1;
 sampler2D _MainTex2;
 sampler2D _MainTex3;
 sampler2D _MainTex4;
 sampler2D _MainTex5;
 sampler2D _MainTex6;
 sampler2D _MainTex7;
 sampler2D _MainTex8;
 sampler2D _MainTex9;
 sampler2D _MainTex10;
 sampler2D _MainTex11;
 sampler2D _MainTex12;
 sampler2D _MainTex13;
 sampler2D _MainTex14;
 sampler2D _MainTex15;
 sampler2D _MainTex16;
 
 void surf (Input IN, inout SurfaceOutput o) {
 
 float x = IN.uv_MainTex1.x;
 float y = IN.uv_MainTex1.y;
 
 bool y4 = (y >= 0.75f);
 bool y3 = (y < 0.75f) && (y >= 0.5f);
 bool y2 = (y < 0.5f) && (y >= 0.25f);
 //bool y1 = (y < 0.25f) && (y >= 0.0f);
 
 float2 product = IN.uv_MainTex1 * float2(4.0f,4.0f);
 
 if (x >= 0.75f) {
 if (y4) {o.Albedo = tex2D (_MainTex4, product).rgb;}
 else if (y3) {o.Albedo = tex2D (_MainTex8, product).rgb;}
 else if (y2) {o.Albedo = tex2D (_MainTex12, product).rgb;}
 else {o.Albedo = tex2D (_MainTex16, product).rgb;}
 }
 else if ((x < 0.75f) && (x >= 0.5f)){
 if (y4) {o.Albedo = tex2D (_MainTex3, product).rgb;}
 else if (y3) {o.Albedo = tex2D (_MainTex7, product).rgb;}
 else if (y2) {o.Albedo = tex2D (_MainTex11, product).rgb;}
 else {o.Albedo = tex2D (_MainTex15, product).rgb;}
 }
 else if ((x < 0.5f) && (x >= 0.25f)){
 if (y4) {o.Albedo = tex2D (_MainTex2, product).rgb;}
 else if (y3) {o.Albedo = tex2D (_MainTex6, product).rgb;}
 else if (y2) {o.Albedo = tex2D (_MainTex10, product).rgb;}
 else {o.Albedo = tex2D (_MainTex14, product).rgb;}
 } 
 else {
 if (y4) {o.Albedo = tex2D(_MainTex1, product).rgb;} 
 else if (y3) {o.Albedo = tex2D(_MainTex5, product).rgb;}
 else if (y2) {o.Albedo = tex2D(_MainTex9, product).rgb;
 } else {o.Albedo = tex2D(_MainTex13, product).rgb;}
 }
 }
 ENDCG
 }
 FallBack "Diffuse"
 }

PS. Sorry for the strange formatting, it seems Mono is not auto formatting on .shader files