The maximum number of guards you can have is 1 guard per 49 tiles. Example: Guard amount of a 31x31 deed is 19.
Guards demand payment. Hiring each new guard will cost 1 2 silver summoning cost. This is deducted immediately from the settlement upkeep, before they spawn. Each guard also adds 30 copper 1 silver to the monthly settlement upkeep.
Guards can be managed at the token under Settlement > Manage guards, or from the Settlement deed paper under Village > Manage guards.