For the number of primary group members of 12 I offer this:
"What Does the Number 12 Mean in the Bible?
Number 12, as
indicated in this article, typically means perfection or authority—often used in a context of government. For instance, because Jacob has 12 sons, they form the 12 tribes of Israel. There are also
12 minor prophets that dot the text of the Old Testament.
Found
187 times in the Bible, the number 12 makes a special appearance in Revelation, showing the authority and perfection of God’s kingdom at last.
We see Jesus chose
12 disciples, and the disciples later replaced Judas Iscariot with Matthias to keep the number at 12 (
Acts 1)—to show the perfection and authority of those who followed Jesus.
Similar to the number three, it’s likely not every instance of the number 12 is a ‘good’ kind of perfection. Jesus heals a woman who had been bleeding for 12 years, for instance (
Matthew 9:20-22). The 12 years could possibly symbolize just how bad her condition was, signifying the malady as a ‘perfect storm,’ with seemingly no possible cure. Or, it could signify that this was completion of a difficult timeframe and God would be doing a new thing now.
However, 12, more often than not, tends to represent something good. The new Jerusalem, for instance, is stocked full of the number 12 (gates, angels, foundations, etc.,
Revelation 21)." (pasted from:
Interesting Facts about the Meaning & Importance of the Number 12 in the Bible)
For the 12 all being men, so far yet, history not having been rewritten here, yet. Jesus clearly loved and cherished woman and did not hesitate to dialog with them, even against extreme cultural taboo, Samaritan woman at the well. He, CLEARLY, was not allowing culture to dictate His choices concerning woman. I do not think the Bible tells us why so I cannot say; but it appears to definitely be an an intentional choosing of all being men as it is also clear that there were many woman followers whom He loved also constantly and faithfully in His midst.