Does anyone have an idea of how many IP addresses max a DOCSIS 3 cable modem can support? I seem to get varying answers even from different people at our ISP.
I'm assuming you mean the number of public addresses that may be either assigned to the modem or routed?
To my understanding, DOCSIS is Layer 2 and below. It shouldn't impose any limitation on assigned IP addresses at all. In the same way, using coax or twisted-pair ethernet, or TokenRing, wouldn't impose a limit on the number of IP addresses on a given port of a device. Hell, you might not even be running IP on a given physical layer, or multiple protocols like IP + IPX + NetBEUI.
Your specific modem may have certain limitations, but without knowing the model, we can't tell you. Your cableco may also have imposed a limit.
Now, if you're referring to a maximum number of internal private-IP addresses that may be NAT-ed, that depends strictly on the router's config, including CPU and RAM, and any provider-imposed limitations as well.