One bike, a $350 Giant Cypress Hybrid. I ride it 4 seasons in Michigan. You do not NEED multiple bikes. What multiple bikes gets you is redundancy in case one breaks, and possibly bikes that may be faster that you can ride when the weather is nice, then the one with full fenders, lights, etc that you can ride other times. I ride the latter all the time.
__________________
Work: the 8 hours that separates bike rides.