It's possible to make a line follower with just 2 photosensors. Such configurations aren't very precise and thus not suitable for high speed. That's why people typically use 4 - 8 sensors.

That's not enough!

The bigger the better! To get even better precision and smooth control, we made an array with 32 infrared reflective sensors. FPGAs have a ton of available inputs, so we can afford 32 parallel channels.

To clean up the signal from our KU163 sensors, we used four 8-channel digital buffers with hysteresis.

The drawback of this approach is that the sensitivity is set by the resistors in series with the phototransistors, which are not easily adjustable.

It works quite nicely!