How can i find data gridview row index in row commend event?

You must be aware of this, that RowCommand event is raised when a button is clicked in the GridView control. Which enables you to provide an event-handling method that performs a custom routine whenever this event occurs.
Refer more detailed description here:
GridView.RowCommand Event[^]

This is how you can get the index of GridView row using RowCommand:

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "Show")
          {
            int index = Convert.ToInt32(e.CommandArgument);
            int documentID = Convert.ToInt32(GridView1.DataKeys[index].Value);

             // Write your further code
          }
    }

Ref.:Get RowIndex In GridView RowCommand Event Using DataKey[^]