We were seven—who now are one, (line 17)
The speaker used to have a large family, but now he's the only one left. Notice how the "one" that's left is even isolated in the poetic line: he's separated from the "seven" by the dash in the middle of the line.
And we were three—yet, each alone; (line 49)
Now the speaker's large family has dwindled to three – yet they're isolated from each other because they're each chained to a separate pillar. Like in the previous example, they're isolated also by the dash in the middle of the line.
We could not see each other's face (line 51)
Even though the three brothers share a dungeon cell, they can't even see each other's faces.