Default similar threads is 5, to change it, do the following:
Step 1: Login to AdminCP > Settings > Options > Thread Display Options (showthread) > Show Similar Threads? > Yes
Step 2: Open file /packages/vbdbsearch/coresearchcontroller.php
Find:
Replace:
Step 3: Login to AdminCP > Maintenance > General Update Tools > Rebuild Similar Threads > OK
Wish you success
Sorry for my bad english !!!