Which is more valued, first authorship articles or the number of articles produced?
As an experienced researcher, I think that this choice is a false dichotomy.

What I look for in a researcher's record is their ability to do meaningful and impactful work, and the strongest researchers in most fields I know of typically have a record that shows a mixture of first-author and Nth-author papers:


If you're capable of innovating and of doing the core work of a project, that should produce some first-author papers.
If your work is impactful and useful to others, that will result in collaborations, many of which will not have you as first author.


Complementarily, I would consider it a red flag to see either very few first-author papers (often indicating poor core research capabilities) or nearly all first-author papers (often indicating poor ability to work with others).

The further along a researcher is in their career, the more that I would typically expect the balance to shift from first-author papers to non-first-author papers, both as there is more time to build collaborations and as they start to supervise more junior researchers (student or otherwise).

To answer your specific question about a postdoctoral application, then, I would typically expect to see at least a few strong first-author papers and also some non-first-author collaborations.  At that stage of career, however, the number of significant papers is typically small enough that strong candidates can have all sorts of different mixtures.  In the end, at least for a researcher like myself, my judgement regarding a postdoc is less about the number of publications than what's in the publications and what you did for them.

Note: the general expectations expressed here may not apply to certain fields with very different publication expectations, such as pure mathematical theory or experimental high-energy particle physics