Update index.md

docs/en/14-reference/12-config/index.md

@@ -722,6 +722,15 @@ The charset that takes effect is UTF-8.
 | Value Range   | 0: not change; 1: change by modification                                      |
 | Default Value | 0                                                                             |
 
+### tmqMaxTopicNum
+
+| Attribute     | Description               |
+| -------- | ------------------ |
+| Applicable | Server Only       |
+| Meaning     | The max num of topics  |
+| Value Range | 1-10000|
+| Default Value   | 20                  |
+
 ## 3.0 Parameters
 
 | #   |     **Parameter**      | **Applicable to 2.x ** | **Applicable to  3.0 **      | Current behavior in 3.0 |