You might want to ask the management or the provider. Unless you're talking about a dorm, I doubt the entire apartment building is going in and out on 1 IP address. Plug something in, and just go to any showmyIP type site. Ask a neighbor to do the same. That alone will indicate if you are on one IP.
Look for equipment. How many switches and ports do you see? How many jacks in your apartment? Multiply that by number of apartments. Does that match up to the number of ports on the switches?
Those are some of the first pieces of info you'll need before proceeding. If it appears that you share the same public IP with the entire building, tell the management that your work requires a secure connection and you can do that with - then tell them what you'll need to do. You don't want to put equipment in or swap cables in a utilities closet/cage that might be under the control of the management or maybe a contractor.
Lots of variables, but have you checked to see what is connected? If there is anything suspicious, you might limit it by MAC address. That said, while smart devices won't put a dent into usage, they'll have separate MAC addresses.