On Snapchat, the people whom you message the most frequently are automatically added to your list of Best Friends. By default, Snapchat sets your Best Friends count to three, but you can change this feature with a simple tweak to your settings.
Set Number of Contacts Displayed in Best Friends List
Open Snapchat > the Settings menu (represented by a gear icon):
Scroll to the Additional Services section, and tap Manage > # of Best Friends:
You can now choose how many Best Friends you would like to be displayed: