ajax function executes the main plugin file

Hello,
Every time I make an ajax request the main file (plugin-name.php) gets rerun,

plugin-name.php

namespace pluginname;
$number = rand(0,10);

require_once(NAMEPLUGIN_DIR.'/ajax.php');

add_action('wp_ajax_randNumber','pluginname\ajax\getNumber');

ajax.php

namespace pluginname\ajax;

function getNumber(){
global $number;
echo $number;// The result varies, 
}

The result varies, because plugin-name.php is executed again.
How can I fix this without using the database?
thank you very much