Zero.
Because person with AB blood group will have (I antigen - antigen on RBCs) has Ia and Ib. While person with blood group O will have only i antigen.
Blood group A: Ia + i ; Ia + Ia
Blood group B: Ib + i ; Ib + Ib
Blood group O: i + i
Blood group AB: Ia + Ib
Probabilities in the given case:
Ia + i = Blood group A
Ib + i = Blood group B.
Thus the children would either have Blood group A or B. Never O.
