Theoretical connected devices would be 255 IP addresses devices connected via wireless and LAN to the router. This is on the LAN side of the router which they can not or should not be able to see or care about. The one problem could arise is how many active WAN side port connections that they can see coming from the ISP Modem and Router. So if your doing any Torrenting, while gaming and streaming videos, this could cause a red flag on there side. Torrenting uses many connections to the ISP to get data UP and Down.
The only device they should see connected to the ISP modem is the router which constitutes one connected device.
How many devices are you connecting and using at a time?