I currently have 5 people connected under my router and we all use the same external IP address. Downside - unless I open up ports and specify things on the router people can't find exactly my machine, instead they see the router as a single computer that sends all the information. So yeah, we ran out, worst case scenario - your'll be wired to another router in the ISP's office/base/HQ/Evil lair and thus you'll need to specify ports and all if you want to host servers. Now other things like downloads, playing games, sending stuff over skype - it'll work juuuust fine.
A guy that's been hiding behind his router for a while now and <3s Hamachi.