Actually that depends on the skill of the user: Tobirama had such precise chakra control that he could do a about 42 hand seal Justus with only one hand seal ...... also on speed and style: Itachi weaved making hand seals and throwing Kunai together and one when he released Amaterasu for the first time onscreen, he used hand seals so fast that Zetsu failed to even see what he was doing....
