Water contains an amount of [H3O]*[OH] (which depends on the temperature).
When really pure, neutrality means [H3O]=[OH]=1e-7. A smaller amount of acid or base just adds ions to this minimum amount - compute it from the known product and from the electric neutrality.
Because water's autoionization is really small, it rarely matters. Tiny amounts of impurities overshadow it. Semiconductor fabs use ultrapure water that is deionized by ion-exchange resins (or was, looong ago), and the theoretical water resistivity is attained exceptionally, under the best circumstances.