I bought an item online and the mailer mailed the item without my apartment number but with everything else included in the address.?

If your apartment building has a manager's office, the parcel will probably 'land' there (if the office is open when the courier tries to deliver). It could also land in the apartment building lobby unattended. (In that scenario, I hope you live in a secure building with trustworthy neighbors who wouldn't steal an unattended parcel that isn't theirs.)

Worst case scenario is that the courier returns the parcel to sender as 'undeliverable as addressed'.