No, not in this sentence. To use "them", you can say "...depends on many factors. Most of them have an effect on one another." but it would be better to use "these", most of these factors. Hope that helps! :)

I don't think changing "which" to "them" is okay. I think it has to do with the connecting clause- "most of which have an effect on one another" . It is unnecessary for the sentence. You could remove it, and still have a functional sentence ("A country's capacity to produce wealth depends upon many factors." is functional.) According to Grammarly, "most of which have an effect on one another" would be a non-defining clause, and therefore, "which" would be the better choice, since "which" is used with non-defining clauses, and "that" or "them" are used with defining clauses. Sorry if this is confusing. I'm a native English speaker, but my knowledge of grammar is not great. I looked this up on Grammarly and that's the best explanation I can give.