It's just historical accident. I doubt the "college professor" cited above knows more than the US Supreme Court.
The Court in Williams v. Florida, 399 U.S. 78, the U.S. Supreme Court said:
"We had occasion in Duncan v. Louisiana to review briefly the oft-told history of the development of trial by jury in criminal cases. That history revealed a long tradition attaching great importance to the concept of relying on a body of one's peers to determine guilt or innocence as a safeguard against arbitrary law enforcement. That same history, however, affords little insight into the considerations that gradually led the size of that body to be generally fixed at 12. Some have suggested that the number 12 was fixed upon simply because that was the number of the presentment jury from the hundred, from which the petit jury developed. Other, less circular but more fanciful reasons for the number 12 have been given, "but they were all brought forward after the number was fixed," and rest on little more than mystical or superstitious insights into the significance of "12." Lord Coke's explanation that the "number of twelve is much respected in holy writ, as 12 apostles, 12 stones, 12 tribes, etc.," is typical. In short, while sometime in the 14th century the size of the jury at common law came to be fixed generally at 12, that particular feature of the jury system appears to have been a historical accident, unrelated to the great purposes which gave rise to the jury in the first place. The question before us is whether this accidental feature of the jury has been immutably codified into our Constitution.
See Duncan v. Louisiana, 391 U.S. 145, 151-154 (1968).
In tracing the development of the jury from the time when the jury performed a different, "inquisitory" function, James B. Thayer notes the following:
"In early times the inquisition had no fixed number. In the Frankish empire we are told of 66, 41, 20, 17, 13, 11, 8, 7, 53, 15, and a great variety of other numbers. So also among the Normans it varied much, and 'twelve has not even the place of the prevailing grundzahl;' the documents show all sorts of numbers - 4, 5, 6, 12, 13-18, 21, 27, 30, and so on. It seems to have been the recognitions under Henry II. that established twelve as the usual number; even then the number was not uniform." The Jury and Its Development, 5 Harv. L. Rev. 295 (1892) (citations omitted).
See J. Thayer, A Preliminary Treatise on Evidence at the Common Law 85 (1898).
Similarly, Professor Scott writes:
"At the beginning of the thirteenth century twelve was indeed the usual but not the invariable number. But by the middle of the fourteenth century the requirement of twelve had probably become definitely fixed. Indeed this number finally came to be regarded with something like superstitious reverence." A. Scott, Fundamentals of Procedure in Actions at Law 75-76 (1922) (footnotes omitted).
1 W. Holdsworth, A History of English Law 325 (1927); Wells, The Origin of the Petty Jury, 27 L. Q. Rev. 347, 357 (1911). The latter author traces the development of the 12-man petit jury through the following four stages. The first stage saw the development of the presentment jury, made up generally of 12 persons from the hundred, whose function was simply to charge the accused with a crime; the test of his guilt or innocence was by some other means, such as trial by ordeal, battle, or wager of law. In the second stage, the presentment jury began to be asked for its verdict on the guilt or innocence of the person it had accused, and hence began to function as both a petit and a grand jury. In the third stage, "combination juries" were formed to render the verdict in order to broaden the base of representation beyond the local hundred, or borough, to include the county.