The maximum possible atomic number of an element is not precisely known, as it depends on the hypothetical existence and stability of superheavy elements beyond the ones that have been discovered. the heaviest known element is oganesson (Og) with an atomic number of 118.
Scientists have successfully synthesized elements up to atomic number 118, but these elements are highly unstable and rapidly decay into lighter elements through radioactive decay. The synthesis of elements beyond oganesson is challenging due to the increasing instability and short half-lives of these superheavy elements.
There are ongoing efforts in experimental nuclear physics to explore the island of stability, a region of the periodic table where elements with significantly longer half-lives might exist. It is speculated that stable or relatively stable superheavy elements could exist around atomic numbers 120 to 126, but this is purely theoretical at this point.
To summarize, the maximum possible atomic number of an element is uncertain, but as of now, the highest confirmed atomic number is 118 for oganesson.