Rounds count, spare magazines-speed strips...
To me, "rounds count"(pun intented)
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It surprises me how many forum members/gun owners have the "hey, so what" or I never carry spare ammunition mindset.
Depending on the uses(field, duty, concealment, etc) & type of sidearm(# of total rounds) I'd opt for at least one fully loaded semi auto pistol mag & at least one speedloader/speed strip for a 5/7 shot wheel gun.
To conceal or carry one pistol magazine won't break you. You may also need a extra pistol magazine in a real lethal force incident.
If the semi auto holds 16-18 rounds like a few newer models that may seem like a lot but a spare magazine should also be handy.
I used a mag holder with my S&W Military and Police full size(17rd 9x19mm).
When I toted my Taurus Protector snub in .357magnum & my DA only SP101 revolver, I had a Bianchi speed strip or 2 with me. Sometimes I used a Altoids mint box,
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To pack 2/3 full size pistol magazines in uniform is becoming more common with armed security & sworn LE. Safariland, Bianchi, Unkle Mikes & Tuff Products all market new slim type 3 magazine holders that reduce the size on the duty belt but allow quick access to the pistol magazine.
Clyde