What are the chances of having a brain tumor when neurologist says everything is fine and the headaches are tension type?

Pretty small: It is pretty unlikely that you have a brain tumor if a board-certified neurologist diagnosed you with a tension headache. The chances are probably less than they are for the general population, which is about 1/50000-100000.

Very unlikely: The vast majority of patients seen for headaches have either tension headaches or migraine. It's probably not uncommon for a primary care physician not to have seen even one patient his entire career who had a primary brain tumor as a cause for headache. A neurologist would generally be cautious and would certainly order an MRI or CT scan of the brain if there was even a slight chance of a tumor.

Unlikely yet keep wa: Headaches are common and rarely due to a brain tumor. Time can help you decide. If your headache are easily controlled over the next few months and there is no other neurological symptoms(vision changes, Vomiting, Seizures etc) then you are not likely to have a brain tumor. Progressive increase in symptoms is how we can tell if there is some growth causing increasing symptoms.