Any of them refers to a group of items or people where the speaker is pointing out that any one of them could fit the description or situation. Synonyms for any of them include any one, anyone, whichever one, whichever, whichever one of them, whichever ones, any, each, every and all. These synonyms can be used interchangeably depending on the context, but they all convey the same idea that the speaker is referring to a group of choices where any one could work. Any of them can be a useful phrase in clarifying that the speaker is not leaning towards any one choice, but considers all of them as viable options.