How can i multiple the custom fields?

Add the HTML for a textarea in the custom box function. You sanitize and save the data somewhere as is done for the input text in the save postdata function.

something going wrong

<?php
/* Define the custom box */

add_action( 'add_meta_boxes', 'myplugin_add_custom_box' );

// backwards compatible (before WP 3.0)
// add_action( 'admin_init', 'myplugin_add_custom_box', 1 );

/* Do something with the data entered */
add_action( 'save_post', 'myplugin_save_postdata' );

/* Adds a box to the main column on the Post and Page edit screens */
function myplugin_add_custom_box() {
    $screens = array( 'post', 'page' );
    foreach ($screens as $screen) {
        add_meta_box(
            'myplugin_sectionid',
            __( 'Home page Projects Detailes', 'myplugin_textdomain' ),
            'myplugin_inner_custom_box',
            $screen
        );
    }
}

/* Prints the box content */
function myplugin_inner_custom_box( $post ) {

  // Use nonce for verification
  wp_nonce_field( plugin_basename( __FILE__ ), 'myplugin_noncename' );

  // The actual fields for data entry
  // Use get_post_meta to retrieve an existing value from the database and use the value for the form
  $value = get_post_meta( $post->ID, '_my_meta_value_key', true );

  echo '<label for="myplugin_new_field">';
       _e("Project Name", 'myplugin_textdomain' );
  echo '</label> ';
  echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field" value="'.esc_attr($value).'" size="25" />';

  echo '<label for="myplugin_new_field1">';
       _e("Project Description", 'myplugin_textdomain' );
  echo '</label> ';
  echo '<textarea type="text" id="myplugin_new_field1" name="myplugin_new_field1" value="'.esc_attr($value).'" size="50" /></textarea>';

 }

/* When the post is saved, saves our custom data */
function myplugin_save_postdata( $post_id ) {

  // First we need to check if the current user is authorised to do this action.
  if ( 'page' == $_POST['post_type'] ) {
    if ( ! current_user_can( 'edit_page', $post_id ) )
        return;
  } else {
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;
  }

  // Secondly we need to check if the user intended to change this value.
  if ( ! isset( $_POST['myplugin_noncename'] ) || ! wp_verify_nonce( $_POST['myplugin_noncename'], plugin_basename( __FILE__ ) ) )
      return;

  // Thirdly we can save the value to the database

  //if saving in a custom table, get post_ID
  $post_ID = $_POST['post_ID'];
  //sanitize user input
  $mydata = sanitize_text_field( $_POST['myplugin_new_field'] );
  $mydata = sanitize_text_field( $_POST['myplugin_new_field1'] );

  // Do something with $mydata
  // either using
  add_post_meta($post_ID, '_my_meta_value_key', $mydata, true) or
    update_post_meta($post_ID, '_my_meta_value_key', $mydata);
  // or a custom table (see Further Reading section below)
}
?>

data not saving and same data showing.

You’re actually pretty close, I think.

One fix is to not reassign $mydata, assign your new field to $mydata1 or something.

The other fix is the add_post_meta() portion is not executable code, delete that line ending with “or” and leave the update_post_meta(... line.

Add a second update line to save your textarea value by changing the key name and the $mydata to $mydata1. That should get both fields saved and the post meta table should have the latest data you entered.