Based on the most recent large-scale and high quality Cochrane review on more than 80,000 healthy adults from 52 clinical trials aged 16 to 65 years, including pregnant women, over a single influenza season in North America, South America, and Europe who received vaccination between 1969 and 2009 (1):
71 healthy adults need to be vaccinated to prevent one of them experiencing influenza, and 29 healthy adults need to be vaccinated to prevent one of them experiencing an influenza-like illness (2)
These statistics (i.e., 71 and 29) are called number needed to be vaccinated/treated (NNV or NNT) representing the clinical effectiveness of vaccines (NNV) or medications (NNT). http://www.thennt.com/ is a quite useful source to find out such measures of clinical effectiveness of diagnostic, preventive, or therapeutic medical interventions.
Considering the difference between pandemics and seasonal outbreaks:
Basically, the current flu vaccines are based on previously recognized sub-types (e.g., in previous epidemics/pandemics). If the virus of an upcoming epidemic/pandemic has been covered in the vaccine, these likelihoods may be true. However, it should be noticed that it is a bit unlikely for a previously well-recognized and covered sub-type to form a pandemic (i.e. an influenza virus that spreads on a worldwide scale and infects a large proportion of the world population [Wikipedia]), unless by mutating to a new pandemic strain. Also, note that studying vaccines' effectiveness against pandemics (the exact protection rate in populations) by current research equipment, methods and budgets is not feasible.
Also, we should not forget that human kind has only experienced 5 flu pandemics.