How are UQ Holder member numbers attributed?
Not sure if this is considered canon, as I stopped reading the manga and am not sure if further details were revealed, but... 

On episode 3 of the anime, when Touta and Kuromaru meet Jinbei Shishido, he tells 'em he's number 2, and Kuromaru says:



Again, not sure if this is canon. Also not sure if Kuromaru is just making an assumption, or if this is a given in the anime — there are some different things in the adaptation, and I don't remember this bit of conversation from the manga. But, if this is not an assumption, then the numbers are attributed according to how powerful the characters are.