How high would my white blood cell count have to be to have leukemia?

Any number. We diagnose leukemia with white blood cell counts that are low, normal and high. Usually the other blood counts are also affected (low hemoglobin, low platelets).
The WBC is always. Elevated in the chronic leukemias - chronic myeloid (CML) and chronic lymphoid (cll). It can be low or very very high in the acute leukemias (aml and all). The number doesn't make the diagnsosis, it is the type of cell and the effect on the other blood elements.