Number of extruders is not an advanced configuration - it is a basic property.

Marlin/Configuration.h

@@ -49,6 +49,9 @@
 #define MOTHERBOARD 7
 #endif
 
+// This defines the number of extruders
+#define EXTRUDERS 1
+
 //// The following define selects which power supply you have. Please choose the one that matches your setup
 // 1 = ATX
 // 2 = X-Box 360 203Watts (the blue wire connected to PS_ON and the red wire to VCC)

Marlin/Configuration_adv.h

@@ -85,9 +85,6 @@
 //=============================Mechanical Settings===========================
 //===========================================================================
 
-// This defines the number of extruders
-#define EXTRUDERS 2
-
 #define ENDSTOPS_ONLY_FOR_HOMING // If defined the endstops will only be used for homing