code: private long getMaxNumFmtId()
      {
          long ret = 163;
          List list = stylesheet.getNumFmts().getNumFmt();
          for(int i = 0; i < list.size(); i++)
          {
              CTNumFmt numFmt = (CTNumFmt)list.get(i);
              if(numFmt.getNumFmtId() > ret)
                  ret = numFmt.getNumFmtId();
          }
          return ret;
      }
docs: Returns the maximum numFmtId in styles.xml.
  @return The maximum numFmtId in styles.xml