Could someone get brain tumor by having several hits to the head?

No: Essentially all cases of tumor associated with trauma is that the tumor is already present and the trauma brings attention to the tumor. Trauma to head occurs in many athletes and fighters and there is no evidence of a higher incidence of brain tumor than normally exists in the genereal population.

No: There's no increase in brain tumors in professional boxers. There has been some talk of increased meningiomas in boxers but even this isn't consistent.