I'm an English learner as well. I read the Bible (NIV version) every day, perhaps I can share how I understand it.
The phrasal verb "tell of" is mainly used in literature, it means to mention or to talk about something.
Since the Bible is considered a classic literature, so there would be a lot of literary phrases used.
While the word "but" in "when they were but few in number" used as an adverb, it means just, merely or only. It would be like: "When they were merely few in number"
Here are some examples using "but" as an adverb:
- What can you say, he was but a child.
- There's no guarantee that I will succeed, I can but try.
Do they make sense to you?