Personal contribution in N-author articles vs single author articles
There isn’t a precise rule about such things, but to the extent that one can formulate a general principle, I’d say that the credit for an N-author article would almost always be greater - often significantly greater - than a 1/N fraction of the total credit. That is why collaboration is generally a profitable activity - it allows two or more people to leverage their skills and abilities and together create something that is worth more than the sum of its parts: the very definition of synergy.

Another synergistic effect in collaborations is that your coauthors will be going around giving talks and telling people about your joint paper. If effect, you get double (or five, or ten times) the “marketing” or “buzz” about your paper for the same effort on your part: the credit you get is higher simply by virtue of more people getting to hear about your paper. Moreover, when your coauthors give talks about your paper they can say flattering things about you that you probably wouldn’t dare to say about yourself, or that if you did say about yourself people wouldn’t be as impressed by (and it works both ways, you say nice things about your coauthors’ contributions when you give talks about the joint papers of course).

I estimate that close to half of my papers are solely authored, and the others are joint. I enjoy both styles of work, although I think these days I most enjoy the freedom that comes with writing papers by myself. I often reflect about the (small) injustice that comes with the fact that those papers usually take longer to make a splash than my jointly authored ones, for the reasons I mentioned above. Oh well. Not every decision we make is driven by some ultra-rational cost-benefit analysis. Do what makes you happy, and what you feel you are good at. Some people excel at collaborating, others are solitary thinkers.

My experience is with pure mathematics, and applies mostly to papers with 1-4 authors. In math, for papers where the number of authors is, say, 6-10 (which are quite a rarity), the credit might start getting seriously diluted, to the point that I’m not sure how much of what I wrote above is accurate - probably it would depend quite a lot on the specific details of who the authors are and what their paper is about.