Most ISP usually don't have visibility on the actual number of devices connected on your home because you are behind a router (that probably runs a NAT that assigns each of your home devices an internal IP).
As far as the ISP can see, there is only 1 connection (via your router) to the ISP. How many devices behind the router is usually not visible. Unless each devices have their own public IP (which is usually not the case).
If the ISP somehow has visibility of how many devices connected to the router and using this information to determine how many devices in your house - as long as your VM network is on NAT mode, it will be fine (as the connectivity is behind your host machine).
If you set it to Bridging mode (where the VM will have its own IP in the LAN) - it will be then detected as the 3rd device as the VM will need its own IP address on the LAN.
Edit (Credit to TheCatWhisperer):
It is incorrect to say that they cannot see how many devices you are
using if you use your own router. Unless ALL your requests are
encrypted, they can simply examine the user agent string sent in most
requests. Whether they would actually go to this trouble or not, who