Can you have too many static functions?

Ive been working with CI a lot recently, and I setup my models so the methods could be used in a static context... Example below:

Code:

<?php

class Accounts_model extends CI_Model
{
   private static $db;

   public function __construct()
   {
       parent::__construct();

       $CI =& get_instance();

       self::$db = $CI->db;

   }

   public static function get_id_from_username($username = NULL)
   {
       self::$db->select('Accounts.account_id')
                ->where('Accounts.username', $username)
                ->from('Accounts');

       $query = self::$db->get();

       return $query->result();
   }
}

And with that kinda setup... since when you load a model in CI, the class is only loaded once, I can create every method as static...

My question, is can you have TOO MANY static methods? is it bad practice? LMK whatcha think. I mean it works fine, since I dont load the classes more than once.. so idk