Typical IP-Address usage per Device

I currently study computer science and got across a question from my prof which he couldn't or wouldn't explain to me. According to my professor one hast to usually plan for 2.5 to 3 IP addresses per physical Device connected to the network. 2 IP address kind of makes sense to me, as there could be some devices like notebooks where a LAN or WLAN connection with a different MAC is possible, so therefore 2 would make sense.
Not calculating in virtual servers, obviously.
I'm interested to see if someone has an explanation for his assumption or if you've made different experiences?