Hi @anish,
1) you need to check with your PCB manufacturer, what is the minimum track and minimum gap what they can manufacture. For critical signals, you may want to increase gap, so signals do not influence each other. I recommend you to find more information about crosstalk topic. To be more specific, normally I do not use gap smaller than 0.1mm (4mil). Maybe have a look at this my post - especially
Rule 6: Be aware of tracks running in parallel:
High Speed PCB Design Rules (Lesson 4 of Advanced PCB Layout). Here you can also see example of table and recommended spacing based on H, where H is dielectricum height between signal layer and reference plane:
2) Please, what stackup are you referring to? If you are referring to the
iMX6Rex stackup, L2 and L11 are grounds:
In case you would like to calculate track with by yourself, you can use an impedance calculator. I recommend you to have a look at
Saturn PCB Toolkit - it's a free software and there are number of useful calculators in one place.