5g orbitals fill during period 8. There are 18 of those so period 8 needs to be 18 elements wider than period 7. Period 7 is 32 elements wide, so period 8 must be 50 elements wide, putting the next noble fluid element at 118+50 = 168.
Something else happens in period 8, though. Electrons in 9s and 9p1/2 orbitals become occupied. These electrons from the n+1 shell make period 8 four elements wider, bringing the next noble fluid element to Z = 172, Usb.
Rather than thinking of ninth period elements interspersed with eighth period elements, it's simpler to think of borrowed orbitals in a continuous period that runs from an alkali metal to a noble fluid. An alkali metal has one electron outside a closed, chemically inert core. A noble fluid is the next core.
Period 8 is the last one which has been quantitatively studied. See Section 4.1 of Wikipedia's article "Extended Periodic Table" for details. It is also the last which can be studied using present tools.
Period 8 ends within a band running from Z = 170 to Z = 175, in which conventional, stationary orbital descriptions become incapable of describing an atom's electrons well. Apparently, orbitals themselves have to move (oscillate) in order to account for the kinetic energy an electron must have to be close to such a charge. (See Sect. 188.8.131.52 in "Extended Periodic Table", Wikipedia.) Beyond period 8, everything is pretty much speculation.