Anything less than a full set from 1-80 will probably be an issue sooner or later.
For pocket watch repair, I would suggest that you start with #3 through #48. These will probably be your most commonly used depending on how you do things.
If you plan to make parts from readily available standard fraction size bar stock, you should then have a few common fraction sizes. For pocket watches, I would suggest 1/16", 1/8" and 3/16" bar stock along with matching collets. This will cover items like screws, Arbors, Pinions etc.
Larger diameters like rachet wheels, crown wheels, barrel covers etc., are best held in machinable pot collets and or self centering chucks.