What's the probability of getting pneumonia or bronchitis from a seasonal flu?

It's possible. Influenza itself can cause an inflammation of the lungs that shows up on xrays, which may be called "pneumonia". It can also injure the cells lining the airways, allowing for a stapylococcal (classically) or other bacteria to infect the lungs. These can be fairly severe processes and may require hospitalization or even ICU care. However this is fairly uncommon.