My reading is that the number of said 'order' is unknown because it refers to the Ainur, not the Istari.
Now, the specification "North of Middle-earth" (UT, IV, ii), may or may not mean that others of that order, sc. Ainur, were sent to other parts. What makes this wholly unlikely, however, seems to be the way the three/five Istari were chosen by the "council of the Valar, summoned it seems by Manwë ('and maybe he called upon Eru for counsel?'), at which it was resolved to send out three emissaries to Middle-earth" (ibid.)
I am not sure to what degree "of whom it is said there were five" (App. B) can be interpretated to mean 'of whom there were allegedly five'. For one thing, it can simply mean that it was already stated as fact, i.e. said, earlier. The other point is that the notes and essays on the istari, which are most important to determining their number exactly and beyond doubt, came later than the Appendices to The Lord of the Rings. Perhaps Tolkien was, at the time of writing the appendices, unsure himself, and later felt the need to remove the uncertainty about the number of istari.
See further
here for an earlier post of mine dissecting the UT istari essay.
[ August 15, 2003: Message edited by: Sharkû ]