Use __init__ function of your class to dynamically add the sales shop to _columns.

class planning(osv.osv):
    def __init__(self, pool, cr):
        shop_ids = pool.get('sale.shop').search(cr,SUPERUSER_ID,[])
        for shop in pool.get('sale.shop').browse(cr, SUPERUSER_ID,shop_ids):
            self._columns['shop_qty_%s'%shop.id] = fields.float(
                                                                '%s Order Qty'%shop.name)
        return super(planning,self).__init__(pool, cr)