I've discussed in other places about this and the conclusion we always came to was that there were only 5 wizards and no more.
Now, for a nice analysis you can read
http://www.barrowdowns.com/faq_istari.php
Now, what I really like to draw your attention to is the difference between ealier and latter texts:
Unpublished essay on Istari from 1954 (later published in the UT):
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the number of the Order was unknown, but of those that came to the North of Middle-earth, the chiefs were five.
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Letter from 1956:
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... the missing two wizards (out of five)
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Another letter from 1957:
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There are five wizards and that is just a unique part of history.
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Also from 1972:
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The other two are only known to (have) exist(ed) [sic] by Saruman, Gandalf and Radagast, and Saruman in his wrath mentioning five was letting out a piece of private information.
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All in all, I personally go with 5, because in later works Tolkien always spoke this way. On the other hand you have an unpublished essay from 1954...which I don't really believe.