There are many factors.
1. Fabrication capability
Smaller spacing will be more expensive to manufacture, and each PCB manufacturer has some limits of their own.
2. Soldering
You will also have to consider your capability of soldering fine pattern PCB.
3. Safety
If the voltage between traces is high, you have to maintain certain clearance.
If the board has to be certified by UL, CSA or similar safety agencies, you need to check with each standard.
If it is just for your experiment in the lab, 0.13mm spacing is okay upto 100Vdc or ac peak. 0.76mm for 300 to 500V.
If it is internal layers of multilayer board, you can use smaller gap.
Hope this helps.
S. H.