1 column to be editable

See the code:

protected void grdPlanning_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        // Set ReadOnly property to true for all columns except the last column
        for (int i = 0; i < e.Row.Cells.Count - 1; i++)
        {
            e.Row.Cells[i].Attributes.Add("ReadOnly", "true");
        }

        // Set ReadOnly property to false for the last column
        e.Row.Cells[e.Row.Cells.Count - 1].Attributes.Remove("ReadOnly");
    }
}

You also need to add the OnRowDataBound event to the GridView control:

 

This code will set the ReadOnly attribute to true for all the columns except for the last column in each row of the GridView, which will be set to false. This will make only the last column editable.