Beyond what volume of added acid is the ph calculated by focusing on the amount of excess strong acid added?

In an acid base titration (for example a strong acid and strong base like HCl and NaOH), at equivalence point, the number moles of acid is equal to the number of moles of base.

HCl + NaOH NaCl + H2O

Moles of Acid = Moles of Base

V(HCl) * M(HCl) = V(NaOH)*M(NaOH)

Now, beyond the equivalence point as more acid is added, the resulting pH will depend on the excess acid present.