With the granting of the 500 troops, now Shizuko is effectively a general (taisho). In the sengoku era, a sonae consist of between 300 to 800 men and is similar to the modern version of military 'company'. Ever played Nobunaga's Ambition? That mixed military unit consisting of pikemen, archers, cavalry and musketmen that you get if you play as a retainer is exactly what she has now! Nobunaga says it's her own private army for her own protection, but he never said she can't bring them to war.
Just offering further trivia for those who are hyped about her going to battle one day. She will also have her own retainers, but let's leave that explanation when the chapter arrives.