The 22nd amendment to the Constitution is : Limitation of Presidential Tenure
Presidents could only serve 2 full terms (8 years)
or
They could serve a maximum of 10 years. If the President were to get assassinated then the vice president could take over for the remainder of the term the assassinated president had. Then they could chose to run for president once they completed the assassinated presidents term. Hopefully that makes sense and was helpful. :)