add arg for num-threads

src/main.rs

@@ -60,6 +60,9 @@ struct Args {
 
    #[arg(long,require_equals=true,num_args=0..=1,default_missing_value_os="true")]
    single_map: Option<bool>,
+
+   #[arg(long,default_value="8")]
+   num_threads: Option<u32>,
 }
 
 #[test]