It should be possible to see that you have 10 machines on your LAN but access to those PCs in better than 99.99% of the time is limited to "you have 10 machines connected to the router."

Now the more paranoid would never run Windows so I'll reveal that when I travel I sometimes boot up TAILS (the OS) from because I can't be sure what is being done on those networks.

If an ISP did misbehave it would be in the news and the end of that company.