If the apartment complex has a resident manager or a rental office on site, an envelope without an apartment number in the address may be delivered there instead. Otherwise, an envelope without this information may be returned to the sender.
If a letter is addressed to an apartment complex without a number, the Postal Service attempts to deliver it to a manager, owner or rental office on site. If a tenant is known, the manager may simply put the letter in the tenant's mailbox or look up the apartment number on a tenant list. The worst case scenario would be that the letter is refused and sent back to the return address as "undeliverable as addressed."