How can we security manage keys for a co-op building?
You may want to consider keyless entry systems. The cards can be programed to allow or deny access. Once a conventional key is issued it will work until the lock is changed. A keyless system can disable individual cards in the event of loss or theft. The initial cost may seem high but if you are rekeying locks and making 48 copies a couple times a year it may be cost effective in the long term. Occupants can issue cards to service or contract vendors and have them disabled if not returned. Some systems can limit entry to specific times only.