The element currently belived to have the highest atomic number (although it is very unstable) is ununoctium. This man-made element has an atomic number of 118 and is located in group 18 (although it is belived to be a "noble solid", not a noble gas), period 7.
A heavier element would have its outermost electrons in the O shell, with its g-subshell holding at most 50 electrons. Since heavier elements have not yet been successfully synthesized, this theoretical value for the maximum number of electrons in the outermost shell remains, well, theoretical.
If you're interested in reading more on the extended period table, you can do so here: