How one and one's is different from other indefinite pronouns The possessive of one (one's) is formed the same way as the possessive of other indefinite pronouns, such as someone (someone's), but it is used a bit differently. For most people, one is consistently used with the possessive form one's.
As @PeterShor points out, in this case "one" is the pronoun, and would never be numeric. Beyond that, as a general rule, spell out numbers 1-9, but for technical writing, it may be appropriate to always use the numeric version when you're referring to a numeral (as opposed to the pronoun example above).
Some people say a dog=one, dogs=ones, the dog=the one=that, and the dogs=the ones=those. It's a rule of thumb, but what I found was that this is not always correct.