Always use the correct key for a clock. Standard keys are available in brass or steel. Generally, keys made from brass are preferable to those made from steel because they are softer. It is a better plan to wear the key down over time than wear the clock arbor. Brass keys are normally fatter than steel ones, because the material is weaker, so you may need to use a steel one, espacially on French clocks where the hole in the dial is minimal.
There are two numbering systems to be found, in addition to the basic dimensions across the flats. The English system increments differently to the Continental one and as keys are often only marked with a number you cannot normally tell which system it refers to. Referring to a key by number is therefore almost meaningless. The old numbering systems have now been replaced by metric measurement of the arbor across the flat surfaces. When a clock is dismantled, examine the flats of the winding arbors and square them off with a file if necessary. Measure across the flats if a new key is needed, or try a bunch of new keys to find the best fit. A loose key will damage and round off the corners of the arbor.
A winding Key Sizing Guage is a useful tool for measuring old keys.