How will the dome return direction?
I would suggest using as many sensors as you need angles to move in, perhaps 8 or 12.
There are input shift registers, I've bought them, so pins should not be a limiting factor.
It could be possible to use 1 sensor and a turning mirror but in that case you have a motor involved and a working sensor sweep would be very slow.
Other things it might need are bump detectors and maybe reflected IR using flashing IR leds that let one IR sensor ring read be passive and the next active.