Using the techniques described in Getting a dump of arXiv metadata, I downloaded metadata for all the math
preprints posted toon arXiv in January 2015, and had a script count the number of authors. I got the following results:
4634251459 papers
10 authors: 15801 papers 34 0.0958%0004%
21 authors: 1636101926 papers 40.5338%
2 authors: 89630 papers 35.3043%6440%
3 authors: 98943290 papers 2117.3423%2155%
4 authors: 31512555 papers 64.7976%9929%
5 authors: 782949 papers 1.6832%1728%
6 authors: 22700 papers 0.4748%2784%
7 authors: 7216 papers 0.0859%
8 authors: 102 papers 0.1511%0406%
89 authors: 140 papers 0.0159%
10 authors: 14 papers 0.0216%0056%
11 authors: 9 papers 0.0036%
12 authors: 7 papers 0.0028%
13 authors: 4 papers 0.0863%0016%
14 authors: 2 papers 0.0008%
15 authors: 3 papers 0.0012%
16 authors: 2 papers 0.0008%
20 authors: 1 papers 0.0216%0004%
22 authors: 3 papers 0.0012%
23 authors: 1 papers 0.0216%0004%
28 authors: 1 papers 0.0004%
37 authors: 1 papers 0.0004%
60 authors: 1 papers 0.0004%
67 authors: 1 papers 0.0004%
I am working on downloadingtook a look at some of the wholeoutliers. Some are ordinary papers but some are other kinds of collective works. Note that some appear to be cross-listed under other arXiv metadata for all time; when that's donesections.
To the asker: There may be some ambiguity here based on what we think of as a "project" and "people involved", and how research is conducted in our fields. As a mathematician, I will updatethink of a "project" as a concerted effort by a researcher, or group of researchers, actively working to resolve a specific mathematical question. If this post witheffort is successful, it normally results in one or more published papers (sometimes three or four, but usually not dozens), and all the researchers who significantly contributed to the solution will be authors of those papers. If it's a two-author paper, then there really were only two people working on that dataproject. (Of course, there could have been other unrelated groups working on the same question, or something closely related, but they would normally publish separately.) So to me, there is a pretty close correspondence between "project" and "paper".
You might also find this question interesting: How do mathematicians conduct research?