The quote you cited suggests that there were more than five who came in the Third Age, so we are still unsure about the time before then.
I shall have to look further into this, but since I have got the swedish version, and it is not always correctly translater (Grrr ) I suggest that someone with an english version look up the exact quote and post it here, so that we can have a discussion based on quotes, which would probably be much easier. Just a suggestion!
Of this Order the number is unknown; but of those that came to the North of Middle-earth, where there was the most hope (because of the remnant of the Dśnedan and of the Eldar that abode there), the chiefs were five. - from 'the esay on the Istari' presented at the beginning of 'The Istari' Section II to Part Four to the Unfinished Tales
And if I remember from non-Tolkien reading, "tū" is French for the insignificent "you", and is usually used by the airistocracy when speaking to servants, serfs, and slaves. So that Tū can be one of the lesser Istari and Old JRRT was pulling our leg with the name of this non-chief Istari.
[Edited on 2/9/2003 by Grondmaster]