Hi and Welcome mateobus,
Set your Canon menu to continuous drive mode (page 73 of your camera's manual), and then make settings for desired type of bracketing in CHDK menu (Main Menu>Extra Photo Operations>Bracketing in Continuous mode), switch out of ALT mode, then shoot desired number of shots by holding shutter button down. Your camera appears not able to shoot more than one shot with Custom Timer Mode.
Hope this helps, Cheers.