This attempt to answer the question "Why 5:5?" falls into four parts: (1) number symbolism; (2) the Matthean context; (3) why ten?; and (4) why 5:5?
Number symbolism in Judaism
(OP) I know that the numbers in Jewish culture have special meaning.
While this is certainly true, it is difficult to see what particular symbolism the number five (or ten, for that matter) holds here in Matthew's gospel. Two convenient surveys of Jewish numerology prompt this observation:
- From the Jewish Encycopedia (1906): the article on "Numbers and Numerology" has nothing at all to say about either five or ten.
- The corresponding article in the Encyclopaedia Judaica (2006), "Numbers, Typical and Important", is more full, and does have an entry for both numbers. Yet in both cases it is only to describe their profiles in the Hebrew Bible; there is no suggestion that they have any wider symbolic significance.
This may not yet be the whole story (see the next section on "ten"), but there is at least no prominent symbolism in the numbers five or ten, or the 5:5 (= 1:1!) ratio. (Perhaps, though, this element of the question would get a much more informed response on Mi Yodeya.)
The Matthean context
Every study of this parable notes that it has many curious, puzzling, even disturbing features. It is also unique to Matthew, and deeply embedded in the "eschatological discourse" of Matthew 24-25. It's certainly worth asking the "why" and "how" questions about the numbers the gospel as a whole uses. Some observations about Matthew itself, then, might help.
The major thing to note here is that Matthew has five "discourses" or blocks of teaching (roughly as follows: 1. chs. 5-7, the "Sermon on the Mount"; 2. ch. 10, the missionary discourse; 3. ch. 13, the kingdom parables; 4. ch. 18, ethical teaching; 5. chs. 23/24-25, the eschatological discourse). Matthew has five uses of the formula "when Jesus had finished saying these things", at the boundaries of these discourses. It seems, then, a very deliberate structure. And one recalls that there were five books of Moses, five books in the Psalms, five "festival scrolls" ... it is very suggestive.
Matthew also likes things in pairs: two "Gerasene demoniacs" (Mark/Luke have one); two blind men by Jericho (Mark/Luke have one); two donkeys at the "Triumphal entry" (Mark/Luke have one, but this might be reflective of a different phenomenon: see R.T. France, The Gospel of Matthew (Eerdmans, 2007), p. 339). This inclination to "pairing" shows up in other ways in Matthew, too -- but this suffices for the moment.
So there is a certain "tidiness" and rhetorical balance in Matthew's presentation overall.
Here, I only report the suggestion(s) of J. Massingberd Ford, "The Parable of the Foolish Scholars (Matt. XXV 1-13)", Novum Testamentum 9 (1967): 107-123 (+see full PDF). Ford argues that the "parable" should rather be understood as "allegory", and in pursuit of this case he draws together a rich background of Jewish texts and practice to inform his interpretation. With this in mind, on pp. 115-6 he writes:
In this parable Our Lord appears to depict, not an ordinary marriage but men who apply themselves to the study of Torah hoping to be led into the bridal chamber, the chamber of instruction.
The symbols are apt. The number of the virgins is ten. Ten is the required number to make up a quorum for the synagogue; ten for the courts of ten in Temple law (Sanh. i. 3); but, most important of all, ten for the circles of study, for example, in the Qumran community (CD. [= Damascus Document] x. 4).
So, why ten? Because it symbolizes the size of group that could gather for Torah study. Ford may be right in making these associations, but this depends to a large degree on the success of applying his whole intepretative framework to the parable/allegory.
It might also be that the pattern of "fives" and "pairs" in Matthew is sufficient to generate "ten", but it would seem somewhat arbitrary. Perhaps the narrative setting of the wedding party makes this a sensible number; while wedding customs of the time must inform our understanding (also the marriage feast of Matt 22:1-14), the ten maidens is left unremarked in treatments I have seen.
Which brings us, finally, to the question of proportions. This seems most readily explained not by number symbolism, but the structure of eschatological judgment as expressed through this discourse:
- 24:29 = [sun + moon] + [stars + "powers"]
- 24:40-41, two in the field, one taken and one left (= 1:1); two grinding, one taken and one left (= 1:1);
- => "you must also be ready" (v. 44);
- 24:45-51 = a "faithful/wise" servant vs. a "wicked" servant (= 1:1);
- 25:1-13 "our" parable, the ten maidens, two groups of five (= 1:1);
- 25:14-30 = three servants: first, has 5 talents, makes 5 talents; second, has 2 talents, makes 2 talents; contrasted with third who has 1 talent, and makes none; the first two are rewarded, the third "cast out";
- 25:31-46 = "all nations" separated into two groups, "sheep" to reward, "goats" to punishment (= 1:1).
The "odd one out" here, it seems to me, is #6 where the structure (two servants v. one servant) is not the 1:1 ratio seen otherwise so consistently. But even here, the pattern of 5+5, 2+2, and the implicit expectation that 1+1 is surely within the grasp of Servant Three, yields a similar sense of balance.
The "eschatological discourse" of Matthew 24-25 culminates in a sequence of descriptions of discriminating those who are "ready" (24:44) from those who are not when the "end" comes. This "bifurcation" -- splitting of humanity into the faithful to be blessed, and the wicked to be cursed -- is recounted in a long sequence of vignettes: figures of speech, parable, allegory, from the pithy to the extended.
In this sequence the "ten maidens" story takes its place with its 1:1 split of the ten women into two groups of five, much like the other "splits" in the surrounding context. Perhaps it figures as part of a move from "set pairs" in ch. 24 (two labourers, two grinders, two servants) to larger groups that culminate in the whole of humanity. Ford's suggestion that "ten" signals a group who would gather to study Torah has some appeal, but cannot be taken in isolation from the larger case he attempts to build.
Further Reading (aside from the commentaries)