MBL can actually do up to 9 x 9

Marlin/SanityCheck.h

@@ -238,8 +238,8 @@
     #error "MESH_BED_LEVELING does not yet support DELTA printers."
   #elif ENABLED(AUTO_BED_LEVELING_FEATURE)
     #error "Select AUTO_BED_LEVELING_FEATURE or MESH_BED_LEVELING, not both."
-  #elif MESH_NUM_X_POINTS > 7 || MESH_NUM_Y_POINTS > 7
+  #elif MESH_NUM_X_POINTS > 9 || MESH_NUM_Y_POINTS > 9
-    #error "MESH_NUM_X_POINTS and MESH_NUM_Y_POINTS need to be less than 8."
+    #error "MESH_NUM_X_POINTS and MESH_NUM_Y_POINTS must be less than 10."
   #endif
 #elif ENABLED(MANUAL_BED_LEVELING)
   #error "MESH_BED_LEVELING is required for MANUAL_BED_LEVELING."

Marlin/configuration_store.cpp

@@ -67,7 +67,7 @@
  *  203            z_offset (float)
  *  207            mesh_num_x (uint8 as set in firmware)
  *  208            mesh_num_y (uint8 as set in firmware)
- *  209 G29 S3 XYZ z_values[][] (float x9, by default)
+ *  209 G29 S3 XYZ z_values[][] (float x9, by default, up to float x 81)
  *
  * AUTO BED LEVELING
  *  245  M851      zprobe_zoffset (float)