City Watch resets are a resource and will accumulate one every 24 hours. You can see a timer on the reset screen to indicate when you will receive the next reset.
The "Banked City Watch Reset" guild perk allows players to hold more than one rest at a time. It does not change how many resets members earn.
When a player leaves a guild that has a "Banked City Watch Reset" perk, those extra resets are removed.