Are you asking about the required minimum spacing between traces?
A definite answer requires you to consider many factors, such as: What are the criteria for success and failure? How thick (not how wide) are the copper traces? Must the board meet certain safety standards? What is the operating temperature, and what is the temperature where the board material starts to deteriorate? Will it be subjected to humid, wet, or corrosive conditions? Will the traces be covered by a coating of insulating material (besides soldermask)?
There are many online tutorials, formulas, and calculators to guide you toward an answer. For commonplace circuits operating in benign environments at typical logic-level voltages, spacings of 0.02mm seem to be more than adequate to prevent electrical arcover. Finding a manufacturer who can actually fabricate such a board is another story altogether!
At the present time, the majority of PCB fabricators specify 0.15mm to 0.2mm (6 mils to 8 mils) spacing as a "routine" job. Some of them will do a board with 0.1mm (4 mil) spacing, but at a higher price. The board fabricator knows what his equipment and processes are capable doing and sets his specifications accordingly. Asking for closer spacing than the specified value runs the risk of shorted traces due to under-etching of the copper material.