<?php
    $meta_box = array(  
    'id' => 'my-meta-box',
    'title' => 'Custom Input Fields',
    'page' => 'page',
    'context' => 'normal',
    'priority' => 'high');

    $fields = array();

    $numberOfArrays = 2;

    for($i = 1; $i <= $numberOfArrays; $i++){
    	$fields[] = array (
    					  'name' => 'Textarea',
    					  'desc' => 'Enter big text here',
    					  'id' => 'textarea'.$i, //id is textarea + number
    					  'type' => 'textarea',
    					  'std' => 'Default value'
    			);
    }

    $meta_box['fields'] = $fields;

    print_r($meta_box);

Array
(
    [id] => my-meta-box
    [title] => Custom Input Fields
    [page] => page
    [context] => normal
    [priority] => high
    [fields] => Array
        (
            [0] => Array
                (
                    [name] => Textarea
                    [desc] => Enter big text here
                    [id] => textarea1
                    [type] => textarea
                    [std] => Default value
                )

            [1] => Array
                (
                    [name] => Textarea
                    [desc] => Enter big text here
                    [id] => textarea2
                    [type] => textarea
                    [std] => Default value
                )

        )

)