EC2 instances can have multiple interfaces in multiple subnets as long as the subnets are all in the same AZ.
Each instance has at least 1 network interfaces if EBS storage is selected as root storage: if EBS Optimization (enabled by default on most new instance types) is enabled a second interface is created (but not visible) to handle storage traffic so that data traffic is not impacted.
EBS Optimization
It adds dedicated capacity for disk operations over the network. Set on a per-instance basis, on or off.
Most instances support it by default and is enabled by default. While older instances offer it at an extra cost.
It is required for some features like IO2 volumes.
Enhanced Networking
Uses SR-IOV: the physical NIC is virtualization aware so that host software doesn’t have to manage translating using system calls.
This allows for higher I/0 (and bandwidth, PPS) and lower host CPU usage and latency.
It’s enabled by default or available at no charge on most modern instance types.