There are none, was just something made up in the 40s, can be taken away

Originally, the office of President had no term limits but around FDR's time the 22nd amendmet made it so a individual could not be elected t as President beyond his 2nd term which is 8 years in total. Now, if trump were to be selected as someones VP pick, he wouldnt be in violation of the letter of the law because of how the amendmet is specifically worded.