This histogram is by default not binned in b-tag multiplicity (and no reason it should). But this means that you have to take care of the correct bins yourself!
If >=1 b-tag is reported you should only use btagweight index > 0, etc.
So for this histogram you need only two weights:
>= 1 b-tag = sum(btagweight index > 0) and
>= 2 b-tags = sum(btagweight index > 1)