Yes, you do need two and it does significantly limit throughput since both the internal and external Ethernet adapters are routed through the same USB bus. The 3B seems to top out at about 90Mbps. I'm not sure what the throughput on a 3B+ would have been, but I'm sure it would have been much lower than its maximum of ~270Mbps.

Having said that, much of the US is restricted to <100Mbps internet and there are some nice advantages to having a relatively powerful little computer running right at the edge.

Any two interfaces. Ethernet+wifi is enough. (Technically, RPi's integrated ethernet port is also connected over USB) I have once seen a single interface both for external and internal network, but that was an abomination.