Really interesting question!! I've never thought about numbers in Tolkien's books (in fact I never think about numbers)But now I'm interested.
As for the Isteri being mentioned in various books, I don't remember anyone but Gandalf mentioned in "The Hobbit". "The Silmarilion" reads:
Quote:
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Of these Curunir was the eldest and came first, and after him came Mithrandir and Radagast and others of the Istari who went into the east of Middle Earth, and do not come into these tales.
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So it may well be that the 2 wizards are reall mentioned only for the sake of number.