Assume one Max7219 unit if not specified

Marlin/src/feature/Max7219_Debug_LEDs.h

@@ -47,6 +47,9 @@
 #endif
 #define _ROT ((MAX7219_ROTATE + 360) % 360)
 
+#ifndef MAX7219_NUMBER_UNITS
+  #define MAX7219_NUMBER_UNITS 1
+#endif
 #define MAX7219_LINES (8 * (MAX7219_NUMBER_UNITS))
 
 #if _ROT == 0 || _ROT == 180