Is it less valuable to participate in a paper with a lot of co-authors? My question is specific to computer science area since in departments like medicine and physics, good quality work usually is a result of big team of researchers.

Not arguing that publish or perish is a fact in academia, I believe there might be some nuances that make an adacemician more or less valuable in terms of preferrability, credibility and reputability.

Publishing quality papers is always a big plus. However, suppose that there are two papers (call them A and B) where A's significance is 8/10 with seven authors whereas B's significance is 6/10 with two authors. And suppose that two people (call them X and Y) have 20 publications each.
X has a lot of instances of A in their CV, and Y has a lot of instances of B.

Assuming that every other parameter is identical (their references, their universities, their field of studies, etc.), which one is more credible? Which one's reference is more valuable? And which one of them is closer to get a the only position in a university, if applied simultanously?
Being first author, or the most significant author in case it is simply alphabetical, is more important than the number of collaborators.  And the impact of the papers is crucial. I'd take someone who is the key author in top venues over someone who had just their advisor, say, but was less important in impact. 

One key question is how the recruiting committee knows the impact of the student on the publications. That's where reference letters come in.