Apparently 3 lines.

But rumors seem to be that the 1st line was where the Russians put most of their effort into. The fact that the 2nd line of defense might already have a breakthrough proves this rumor.

The 3rd line of defense may be worse defended than even the 2nd line. So Russians may be failing at elastic defense (the 2nd and 3rd lines are supposed to be harder to take, because you can move troops and "react" to how the enemy works vs the 1st line). So this is a very promising development, if the lines get "easier" instead of "harder" to breakthrough, then the 2nd and 3rd lines almost don't count at all.