[Legate of Amon Lanc]

Actually, it's even more interesting: not to other parts of Middle-Earth, but the quote you provided implies that there were more sent to the North, and among them the chiefs was five. From other things that are said about Wizards, Valar certainly had main interest in supporting the resistance in the NW Middle-Earth, as there the resistance against Sauron was the greatest, so maybe it is even more probable that more Wizards were sent to the North than that there were even any sent, let's say, to Harad (though I'd find even that logical, as "support of resistance" would be surely helpful even there as "infiltrators" - nevertheless, the Wizards were forbidden to use too much of their power, so they surely wouldn't be that efficient). But anyway, as the quote speaks about those sent to North as a group, it could be concluded that there was another group in contrast to this one, and that was sent elsewhere.

One personal interpretation here. It is playing with the text a little, but when I was making an RPG setting for late 3rd-Age Middle-Earth and one of my players chose to be a wizard (though an Elf), we used the hazy language of this text to our advantage and, though he was simply an Elf from Middle-Earth, of course with no particular contact with Valar or anything, we in fact placed his character under the group labeled "Istari" as one of these "other" Wizards in the North. What allowed us to make this move: the text does not necessarily say that all the Wizards had to come from Valinor, and it does not say explicitely in which way they were "called" for their assignment. It was balancing on the edge, yet we interpretated it the way that this wizard character, having a vocation from Valar - but not any "physical" one, rather a kind of "inspiration", he probably did not even consciously know he has this assignment from Valar - would be, at least in the eyes of the historians several hundred years later, labeled as one of the Istari.

I wanted to use this illustration to show that the borders could be somewhat hazy and that not all is gold that glitters, respectively, in this case rather not all that is gold has to glitter.

Back to something concerning the topic in general: There are other quotes, however, that deny that there will be any other Wizards than the Five. Nevertheless, I consider what you posted an important piece of evidence that should be taken into account and not simply dispatched.