According to the article, the original data was provided by the AMS. I don't think that the AMS leaves this sort of data lying around on laptops on trains, so do to it again you'd have to go and ask them. I suspect that, quite reasonably, the AMS like likes to know what uses their data is put to.
On the other hand, data can be mined very easily from the arXiv via the API. I don't know if arXiv data would suffice for you. If so, a little scripting showed the following data for the month of October:
math.CO: 36, 38, 11, 7, 2, 1
math.CA: 9, 11, 5, 1, 1
math.CT: 4, 4, 3
math.GN: 6, 7, 2
math.AT: 18, 9, 2
math.AC: 6, 9, 4, 2
math.CV: 24, 16, 1
math.OC: 6, 11, 5, 3
math.MG: 7, 7, 4, , 1
math.HO: 14
math.DG: 43, 48, 16, 3, 1
math.LO: 9, 2, 2
math.RA: 12, 11, 7, , , 2
math.ST: 3, 14, 2
math.PR: 43, 45, 25, 7
math.GT: 29, 22, 4, 2
math.SG: 13, 4, 2
math.GM: 8
math.SP: 7, , 4, 1
math.FA: 22, 18, 9, 4
math.OA: 9, 6, 4, 3, 1
math-ph: 52, 53, 15, 2, 1
math.DS: 23, 17, 9, 2, 1
math.QA: 13, 13, 3
math.KT: 3, 1
math.GR: 17, 27, 3, 2
math.NA: 4, 13, 9
math.RT: 28, 14, 3
math.NT: 48, 31, 8, , , 1
math.AP: 49, 59, 29, 4
math.AG: 69, 34, 11, 1, 2
Total: 634, 544, 202, 44, 10, 4
Average: 0.44, 0.37, 0.14, 0.03, 0, 0
The ordering is by number-of-authors. So for math.KT there were 4 papers, of which 3 were single authored and 1 with 2 authors. Missing entries are 0s (so in math.NT there was a 6-author paper but none with 4 or 5). So collaborations outweigh single-author papers by a little bit (technical term).
