How to refer to one occasion of an interviewee talking?

Since the "occasions" are specifically referring to times when a question was asked and answered, I would think that you can replace occasion with either

"question" (M-W)

an act or instance of asking

or

"inquiry" (M-W)

a request for information

• the interviewee is prompted to speak by your questions. When you refer to the talking performed by the interviewee, it is in the form of a

response (dictionary.com)

an answer or reply, as in words or in some action.

Therefore, the resultant sentence would read

B responded to 45 of 399 inquiries/questions

Although you could consider “contributions,” for participants are arguably contributing/making contributions to the focus group each time/occasion they say something, I would reserve “contribution” to refer to, describe, and evaluate their total, overall contribution (or lack thereof) to the group, and refer to each individual occasion as an “utterance”; …“statement” ; … or “remark”, with "remark" being, in my non-academic opinion, the most idiomatic of the three to use in the blanks in your examples:

"in remark X, B said that ..."
"B made 45 of [the] 399 remarks."

Definition of utterance (from Merriam-Webster)
1 : something uttered; especially : an oral or written statement : a stated or published expression

Definition of statement (from Merriam-Webster)
1 : something stated: as
a: a single declaration or remark : assertion

Definition of remark [countable noun] (from *Oxford Learner’s Dictionary)
something that you say or write which expresses an opinion, a thought, etc. about somebody/something

I think "comment" would work perfectly here.

Comment
noun
1. a remark, observation, or criticism