Prime is correct. If he serves for less than 2 years after replacing Biden, he could be elected for a second time and serve 4 more years.
Prime is saying the dems would force Biden out so Trump would be forced to serve more than 2 years and therefore ineligible to run in 2024. (surely Trump would resign 1 day before he hit the 2 year mark though, right?)
Here's the constitution:
Section 1. No person shall be elected to the office of the President more than twice, and no person who has held the office of President, or acted as President, for more than two years of a term to which some other person was elected President shall be elected to the office of President more than once. But this Article shall not apply to any person holding the office of President when this Article was proposed by Congress, and shall not prevent any person who may be holding the office of President, or acting as President, during the term within which this Article becomes operative from holding the office of President or acting as President during the remainder of such term.
Section 2. This Article shall be inoperative unless it shall have been ratified as an amendment to the Constitution by the legislatures of three-fourths of the several States within seven years from the date of its submission to the States by the Congress.
In short, the constitution says if you have served more than two years in a presidential term in which you were not elected president, it counts as you serving one term as if you were elected.
If Trump was elected VP, he should pardon himself, his family and his businesses to stop him from being the target of more politically motivated prosecutions. The fact that he has not broken any laws does not matter. There is even an argument that he should do this before he leaves office, however, I see it being less likely he will be the target of a politically motivated prosecution after he is no longer president because doing so will do very little.