multiple sources vs. one source only
-- early source vs. centuries later
. . . 'single source' as in, we have only a single source for the miracle stories of the Old Testament, such as the Tower of Babel, the Noahic deluge, and Joshua's stopping the earth from rotating. The earliest copies of those documents date back to the Babylonian exile.
So it makes sense to say there is less evidence for these earlier "miracle" stories, whether they might be true or not -- in any case, a critical skeptic has more reason to doubt these ancient legends ("legends" at the time our account(s) of them was/were written), whereas for the miracle acts of Jesus we have 4(5) accounts very close to when the event(s) in question reportedly happened (and thus not ancient legends when our sources were written), making these events more credible, to a critical skeptic seeking the truth about what happened.
It's significant WHEN the source was written and HOW MANY sources there are. Where there are multiple sources (rather than only one) and these are closer to the reported event(s), the claimed event is more credible. There shouldn't be much disagreement on this point.
. . . one small set of miraculous stories in the Bible (namely, a Resurrection) must be true because we have "four or five" sources that were written "very close" (i.e., forty to sixty years) of the event, and . . .
More correctly, 20 to 70 years.
Yes, "very close" by comparison to most (ancient) historical writings, the sources we rely on, which report events typically 50-200 years later than when the events happened (or you could argue 50-100), and especially by comparison to most or all reported
miracle events, which are reported typically many centuries later than the event allegedly happened. (Events reported in a source contemporary to the event are rare exceptions to the norm.)
. . . and they more or less agree (ignore the differences down to stylistic flourishes, please.)
What the multiple sources agree on is more credible, whereas the details where they differ are less credible, or more doubtful. The reasonable conclusion is to accept the general points they agree on, as having high probability, but set aside the doubtful details.
So, e.g., for the Resurrection of Jesus, it's reasonable to conclude that the place where he was buried was found to be empty, the body missing, but it's doubtful who discovered it first, who or what they encountered there, what words were spoken to whom, etc. Likewise probable that he was encountered alive later by several witnesses (all the accounts agree), but exactly where the encounters happened or who said what, etc., is doubtful (less agreement between the accounts).
Counting authors in a collection of works is not very compelling in evaluating the veracity of the works.
What "collection of works"? the New Testament? That didn't exist at the time in question, the 1st century. All written accounts relating to the disputed claims should be counted. Where the number of sources is greater, the reported events are more credible.
Just because the writings are gathered into a "collection" by later editors or scribes or ecclesiastics does not make them less credible than other writings not gathered into such a "collection" by someone 200 or 300 years later.
Otherwise, a collection of ghost stories that has "forty unique authors!!!" would also have to be accepted as true.
No, there's much in the "collection" that can be doubted or disbelieved as not true in those cases where the authors contradict each other. Each source or author is judged according to how much his account is corroborated by other sources, not by whether it's part of a "collection" someone compiled.
If those ghost story writers all agree on some point of fact about something that happened, all saying it's true (not fiction), then it adds credibility to their claim that it happened, regardless of other details they disagree about. Maybe they're all reporting a real event where witnesses did see something weird. This weird phenomenon then becomes more credible, whatever the explanation for it. This credibility is not then undermined because someone later collects the accounts into a "collection" about the particular subject matter.