Plugin set default setting value when it activated

I have set up my plugin option on admin panel with

   /**
     * Register and add settings
     */
    public function page_init()
    {        
        register_setting(
            'my_option_group', // Option group
            'write_here_options', // Option name
            array( $this, 'sanitize' ) // Sanitize
        );

        add_settings_section(
            'setting_section_id', // ID
            'Set Edit Page', // Title
            array( $this, 'print_section_info' ), // Callback
            'write-here-setting' // Page
        );  

        add_settings_field(
            "pid_num", 
            "Select Page >>", 
            array( $this, 'wh_select_list' ),  
            "write-here-setting", 
            "setting_section_id"
        );

        add_settings_field(
            'num_of_posts', // ID
            'Number of Posts to show', // Title 
            array( $this, 'num_of_posts_callback' ), // Callback
            'write-here-setting', // Page
            'setting_section_id' // Section           
        ); 

    }

So in DB, my plugin setting saved in wp_options table under column name option_name as write_here_options in my case as object.

When people activate the plugin, I want to save default values in the DB for pid_num => 0 and num_of_posts => 10.

How do I make this work??

Your code will certainly use get_option() to retrieve the values for your options. get_option() accepts a second argument that allows you to specify a default. Use that instead of inserted values into the database unnecessarily.

get_option( $option, $default );

If you are concerned about third party code, there is there is the option_{$option} filter that you should be able to use to preserve your default even then:

116         /**
117          * Filter the value of an existing option.
118          *
119          * The dynamic portion of the hook name, `$option`, refers to the option name.
120          *
121          * @since 1.5.0 As 'option_' . $setting
122          * @since 3.0.0
123          *
124          * @param mixed $value Value of the option. If stored serialized, it will be
125          *                     unserialized prior to being returned.
126          */
127         return apply_filters( 'option_' . $option, maybe_unserialize( $value ) );