Correct, the N300 router will "route" the WAN traffic to the cable modem.
Cable Modem Ethernet -> N300 WAN port
-> N300 Wireless Signal -> PC WiFi
And/Or
-> N300 1-4 Ethernet Port -> PC Ethernet
Thus allowing multiple devices to connect to your internet service. (Smart Phone, Smart TV, tablet, laptop, PC, etc)
I believe most consumer electronics support up to 32 concurrent devices connected via WiFi depending on Makes/Models.
Don't forget to configure your N300 WLAN (wireless local area network) with WPA2-Personal security and a Passkey.
I prefer using the MAC filter as well to prevent unwanted access to my wireless network.
This way someone can't brute force the password to connect to your wireless network as the router will only allow registered MAC addresses to connect.
You will not incur additional charges.
As a side note, some ISPs (internet service providers) do not provide support for setting up/troubleshooting a router to the cable modem, but should be able to also answer your question about additional charges as well. What I mean is if you have issues connecting to the internet and call the ISP, they will not troubleshoot the connection to the N300 router, they will only provide support for the connection to the cable modem and to the PC.