On a cbc, what things might set off a red flag to a dr that a patient might have leukemia? Wbc or rbc?

Usually WBC. Leukemia is defined as an increase in wbc, however, some pre-leukemic conditions can start with increase in rbc or platelets.
Leukemia changes. Leukemia can be associated with various changes in the CBC such as high or low wbc, low or sometimes elevated platelets and usually anemia (low rbc) rather than high rbcs. The diagnosis almost always requires a bone marrow biopsy be performed as there are many reasons for changes in the blood counts aside from leukemia.