"Of this Order the number is unknown; but of those that came to the North of Middle-Earth [ ... ] the chiefs were five."

To my understanding this either means:

a) that the only the masters of the Order came to ME while the majority of its members remained in Aman, or;
b) that an unspecified number of Istari (one would suppose more than six) came to ME under that guidance of these five.

Either way, 'therein lies a tale', so to speak.

Any ideas?

Cheers
Berembrôg