I have never seen a number spec'd out before, but I'm sure there is a threshold # that the drive triggers on for cleaning purposes. I believe there are alot of factors that can cause soft errors, the condition of the tape for one. New tapes will actually produce more debris on a head than older tapes and may cause more soft write errors. I wouldn't worry to much about the soft errors until you start seeing your throughput decrease. I looked at 8 of my SDLT320's and they range anywhere from 2K to 10K soft errors.