If a solution has a perfect neutral pH, what is the concentration of hydrionium ions in the solution

Neutral pH means a pH of 7, yes? Recall that 'pH' means the POWER of Hydrogen. You should be familiar with the equation : pH = - log[H+].

In this case, 7 = log[H+].

Mathematically rearranged, this means that to find H + concentration,

[H+] = 10 ^-pH.

In this case, [H+] = 10 ^-7.

I am assuming this is conducted at a standard 25 degrees. The auto-dissociation formula of water is this: Kw = [H3O+] x [OH-] at 25 degrees.

The concentration of Hydronium is the same as the concentration of hydrogen, 10 ^-7. mol/L.

There's your answer!