Hey there,
It really depends on so many factors - firm, location, position, etc...
In my experience, ~25% pass first round, and ~33% pass final round - so overall about 8% of candidates pass the interviews after passing the CV screen.
I wouldn't dwell on this, though - whether the percentage is 1% or 50%, MBB offices all have very high standards for applicants. It all comes down to your prep and your previous experiences.
Good luck!