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

There is no answer that is uniform across academia.

In many fields, such as pure mathematics and much of computer science, paper authors are listed alphabetically and being first author is an accident of birth rather than anything else. Jacob Aagard would be first author on almost any pure maths paper he wrote; Admiral Elmo Zumwalt would be last.¹

As I understand it, there are fields where most papers are single-author so, again, the question becomes meaningless.

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¹ I was once at a workshop in honour of Moshe Vardi. Jeff Ullman's talk included a slide about why he likes working with Vardi: 1. He's a nice guy; 2. He's super-smart; 3. His name comes after mine in the alphabet. Vardi stands up and says something along the lines of, "That's very kind. I work with Pierre Wolper for the same reasons."

First, you are talking about at least three dimensions:

1. Contribution: In most but not all disciplines, first authorship often denotes greater contribution. Other indicators include being one of a smaller list of authors (e.g., being second author on a two author-paper compared to a ten author paper).
2. Quality: The quality of your papers; this can refer to the actual quality of the paper as judged by experts who read it; equally, some will judge quality based on the reputation of the journal or other indicators such as citations to the article.
3. Quantity: Number of papers.

You could also contrast total career output versus recent average annual output. Total career output speaks somewhat to your overall reputation, whereas average annual output speaks to how productive you are both in general, and in recent years. This distinction is more important when people are comparing output of researchers with varying career lengths. In the case of comparing graduating PhD students for post doc positions, this distinction is often less relevant.

More generally, there is substantial variation in how contribution, quality, and quantity are weighted to evaluate you as a researcher.

In general, people evaluating you want to see lots (high quantity) of high (or at least good) quality papers with a decent number of first authorships, and see that this productivity has been sustained in recent years.

Of course, the amount of your time required to generate a given output is broadly related to the value assigned to it by people evaluating your research. On average, higher quality papers where you are the lead author take more time to write than lower quality papers where you are playing a support role.

Given the huge variation across disciplines, countries, universities, academics, it's difficult to give general advice. But here are some general comments.

• Try to get a few good first-author papers. But also get involved with other people's papers. This shows that you can collaborate. And in the cases where "number of papers" matters, playing a support role can speed up the process.
• Ask around to get a sense of the relative importance of journal rankings / impact factors / journal reputation in your area.