The amount of steps you take in one mile doesn't really depend on Fitbit but instead the amount of steps you take in one mile depends on how fast you walk.
But most people who walk at an average walking speed takes between 2,000 to 2,500 steps in one mile when walking.
When you run you tend to take less steps and instead of 2,000 to 2,500 steps you normally take 1,400 to 1,700 steps when running.
But according to my Fitbit tracker I take 2,000 steps in one mile of walking and if I walk faster I could likely take 2,500 steps.