There are a lot of misconceptions out there..on both ends. It's common that ISPs say "windows" when they really mean "a computer", and if they don't have manuals for something (usually anything but Windows and their own software, if any), they say it's no good.
Shortly: they are just offering a gateway to the internet. It comes to you by some means, at your end the only thing that matters is how you connect your hardware to the line (RJ45 cable directly to the wall maybe, or a router between your computer and the wall). The connection doesn't know, or care, what's at the ends - and the ends don't care what's in between, as far as the data transfers ok. Therefore it doesn't matter what machine or operating system you use, as far as you can connect the hardware pieces so that data can be transferred, and you have some piece of software that controls the transfer. Linux (any distribution) does this, and so do personal computers in general.
What most ISPs (all?) mean when they say you can connect N machines simultaneously, is probably that you can connect and get so many internet-visible ip addresses or something. If you just "can connect one", like the case usually is, it means your ISP provides you one external IP (that is visible on the net), and that is the IP of your router (or your computer, if it's directly connected). If you happen to have a 10-port router home, that you can connect to the cable and have it open the connection for you (thus getting that one external IP for itself), it can then create a subnet at your home for your several computers that get an internal IP (from the preserved IP range) that's only visible between your home network, and not to the outside world (the internet). To the outside it just looks like one IP (your router) is transferring stuff, and inside it looks like your router is asked to get something, it gets it and puts back to whoever (computer) asked it. There are pros and cons in this, but shortly said unless your ISP requires a special software to connect (for example they aren't ok with the regular standards, but want some extra identification thing), the connection works on anything you can get to understand an ethernet cable.
I don't think anybody in their full senses would track MAC addresses; MAC addresses only need to be unique within the same subnet they're connected to, so connecting several computers to a router which connects to the net means there's no sense in trying (if even possible) to "see" the MAC addresses behind the router. There can be, and probably are, several identical MAC addresses, but never in the same subnet.