Very small number. The typical watch were exactly typical "typical soldiers." Most wore leather aprons or mail shirts and carried a stout club. They were better than conscripts... your conscripts were basically 1st level Commoners with decent ability scores with a sprinkling of 1st level Warriors.
Yeah, but I would distinguish (as 1e does!) between
(a) civilian Watchmen, analogous to the modern 'security guard' or, at the high end, small-town policeman and
(b) City Guard - Soldiers permanently employed by city states (mostly in Italy and Germany) as the first line of defence and the core of the city's protection. Funded by the Guilds, only the very best veteran mercenary infantry (eg from the Landsnechte) could hope for permanent employment as such. If the city levied a citizen militia or hired individual mercenaries ad-hoc* in war time then the Guardsmen would act as Serjeants or elite units in the city's army.
*As opposed to hiring an entire mercenary army, as was common in Italy especially.
Edit: In an RPG setting, not all cities will have this sort of City Guard. A city deep within a powerful state will not, though it may have Royal garrisons or a noble's troops in residence. But a wealthy trade city like Greyhawk certainly should. The city relies on these guys to defend it! A lot of 1e-era cities especially have completely worthless 0-level guards, combined with typical detailed NPCs being 5th level Fighters and such.
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