Note: the Lx ones are emitters and the Tx ones are detectors
It would be better naming them LEDx and either Qx, Trx or some other established convention (and not using an LED symbol for the photo transistor/diode).
1-is it right?
Values are often quite open for interpretation/debate, but I probably would change R7
And I'd commit seppuko before drawing the opamps with their ground leads pointing upwards.
2-can I use 1 potentiometer instead of 2?
Yes, given that all the photo transistors are identical in their response, the op-amps (used as comparators) only needs a common reference. I would add a cap from wiper to ground though (around 1µF).
You could make a similar circuit from a single 8 line buffer, by halving R1, R3...R15 and using a single potentiometer for setting their shifting points.
3-which one is better to use for the pair of ir emitter-detector?
0 0 or 0 0 0 0 0 0 0 0
That really depends on your chassis and your steering strategy.
If you're using a 3 wheel setup with differential steering, mounting them on an imaginary circle with its center right between the two steering wheels, they will each have the same angular coverage when the wheels are turned and it will thus be usable for direct steering.
The same counts for steering made like on a soap box racer (where the entire front axle pivots around its center.
If you use Ackermann steering (like on a real car), a line, or at least a very shallow curve, will give the best spread. Alternatively they can be mounted with uneven center distances.
It's all about getting the most (i.e. the optimum amount of steering) info out of each, so you need to examine your steering for that.
Given that you probably need most precision when going straight and can live with a more coarse indications when turning, you might wanna consider a spread that gives you that, to avoid a zig-zag pattern when going on a straight line (the tell-tale of poor mechs and/or poor programming).
The further forward you can mount the line sensor, the more time for braking and turning, but if they're not under the vehicle, remember to shade them by other means, or ambient light may render them blind.