Realistically, no one will give their officers only 1 spare magazine. I suggest 2 magazines, partly because of that. More than one spare is carried because you WILL reload more than once.
From a gameplay standpoint, the officer is unable to reliably engage more than once. One mag would really disappear in a blink, and a second mag would only allow the officer to safely retreat. Not very good when you're the one with no ammo left. It could've been partly negated had the guns been able to fit 14 shot mags, but unfortunately, it is not
So give every officer 2 spare mags by default, please.