Depends on what you want to pay! You want to look at the pcb manufacturer capability list. The 'usual' for the regular services is 6thou track and space. In metric this is 0.152mm (1thou = 1/39.37mm). For more advanced pcbs, you can go to 4thou track and space, but you generally pay more. You also need to consider the minimum hole size (or drill) and minimum annular ring - this is the size of the circle on the via and pads. Again, refer to the pcb manufacturer capabilities. In general, try to make your tracks as large as possible - it makes it easier to fix any problems you might have. If you are doing a pcb with through hole ics and components, you might make your tracks 0.3mm. If you are using small pitch smd components, then you might have to go to thinner tracks. With your other post regarding current capacity for tracks, there's online sites that have calculators for this. Again, consider this the minimum and make your tracks as fat as possible. Also consider the inductance of the tracks - especially with switched mode power supplies. Most microcontrollers these days are high speed, so proper bypassing with capacitors and low inductance is required. The pcb is an active component of your design rather than just an interconnect.

Have a look at: https[://jlcpcb.com/capabilities/Capabilities](https://jlcpcb.com/capabilities/Capabilities)