Sonny had three mags for that Bren. In one episode he does a bit of time at a Hogan's Alley type range and you can see him use mags from both pouches.
In a scene in the Everglades, his reload is with a partial mag (first time I ever saw anyone do that on TV) and puts one in his mouth to prevent dropping it.
I remember thinking: This is why shoulder holsters are impractical.
Of course when I was only a little older I went out and bought a shoulder rig like Sonny's.