They're in LEO which means approximately 15 minutes of visibility (horizon-to-horizon). The specific time will vary based on the orbital elements but 15 minutes is a good rule of thumb. To maintain coverage you need there to be some overlap in their visibility for a location. There's also a limit to how many connections each satellite can support.

Not all the satellites that you can see will be "looking" in your direction for a signal. They support some number of cells (specific, small, geographic regions on the ground). No one satellite can cover the entire ground visible to it while overhead so more satellites are needed.

And to add to the above, Starlink is using laser crosslinks to connect their satellites to each other for routing. This crosslink network is improved with more satellites visible to each other.