From http://wordpress.stackexchange.com/questions/129180/add-multiple-custom-fields-to-the-general-settings-page

Add multiple custom fields to the general settings page.php

add_action('admin_init', 'my_general_section');  
function my_general_section() {  
    add_settings_section(  
        'my_settings_section', // Section ID 
        'My Options Title', // Section Title
        'my_section_options_callback', // Callback
        'general' // What Page?  This makes the section show up on the General Settings Page
    );

    add_settings_field( // Option 1
        'option_1', // Option ID
        'Option 1', // Label
        'my_textbox_callback', // !important - This is where the args go!
        'general', // Page it will be displayed (General Settings)
        'my_settings_section', // Name of our section
        array( // The $args
            'option_1' // Should match Option ID
        )  
    ); 

    add_settings_field( // Option 2
        'option_2', // Option ID
        'Option 2', // Label
        'my_textbox_callback', // !important - This is where the args go!
        'general', // Page it will be displayed
        'my_settings_section', // Name of our section (General Settings)
        array( // The $args
            'option_2' // Should match Option ID
        )  
    ); 

    register_setting('general','option_1', 'esc_attr');
    register_setting('general','option_2', 'esc_attr');
}

function my_section_options_callback() { // Section Callback
    echo '<p>A little message on editing info</p>';  
}

function my_textbox_callback($args) {  // Textbox Callback
    $option = get_option($args[0]);
    echo '<input type="text" id="'. $args[0] .'" name="'. $args[0] .'" value="' . $option . '" />';
}