The fifth shell of an atom
The $2n^2$ rule is nothing but a mathematical consequence of the mathematical description of electrons around an atom – what chemists call quantum theory and is currently the most accepted description of atomic matter by a mile. Thus, until quantum theory is significantly amended, the fifth shell does, in theory, have the capacity to hold $2\times 5^2=50$ electrons.

I say in theory, because shells aren’t filled all at once bottom to top. In fact, the $n^\text{th}$ shell has $n$ subshells denoted by the letters s, p, d, f, g, h, … which in neutral atoms are (usually; there are exceptions) filled in a specific order known as the aufbau principle. For the first two shells this doesn’t have any effect, but the 4s shell is filled before the 3d shell and the 6s shell before the 4f shell, to name just two examples.

To completely fill the fifth shell (i.e. fill the 5g shell), we would need to also fill the 6s, 6p, 6d, 7s, 7p and 8s shells. The heaviest known (i.e. synthesised according to the standards laid out by IUPAC and IUPAP) element as of 2019 is oganesson (Og) which also happens to be the last 7p element. Two heavier elements (the 8s elements) must be discovered before the third in line will most likely be the first 5g element. The g subshell can hold 18 electrons, so until we have one with a completely filled 5g shell some time will pass.