Hellow, When I use an automatic method for S0, how can I get the exact final value (2 or 0.1 or other) which SAM had calculated ? Thank you very much!

Assuming you mean the CRAN package samr, you are in the wrong place. This support site is for Bioconductor packages, not CRAN.

In addition, you can answer that question yourself by simply reading the help page for SAM, which tells you that part of the output is the output from samr, and the help page for that function should be instructive.