NLB reservation

The Network Load Balancer (NLB) automatically scales to handle your workload without requiring capacity reservations. The Load Balancer Capacity Unit (LCU) Reservation feature is not about increasing the maximum capacity of an NLB, but rather about setting a guaranteed minimum capacity for anticipated traffic surges.

NLBs start with three Gbps of capacity and can scale up at a rate of three Gbps more per minute as your traffic increases. This automatic scaling happens regardless of whether you have an LCU reservation in place.

The LCU reservation feature is designed for scenarios where you need immediate access to a higher capacity level without waiting for the automatic scaling to catch up, such as during event ticket sales, new product launches, or when releasing popular content.

When determining capacity needs, you can convert your traffic into Megabits per second (Mbps) using the ProcessedBytes metric with this formula: ProcessedBytes(per min) * 8/60/(10^6), and then divide by 2.2 to get the LCU equivalent.

There isn't a specific maximum limit for "processed Gb/second" documented for NLBs without reservation, as the service is designed to scale automatically to handle millions of requests per second based on your traffic patterns.
Sources
Using Load Balancer Capacity Unit Reservation to prepare for sharp increases in traffic | Networking & Content Delivery
Load Balancer Capacity Unit Reservation for Application and Network Load Balancers - AWS

The capacity of the NLB can scale to millions of requests per second, regardless of the LCU reservation.

The LCU reservations refers to the minimum provisioned capacity of the load balancer and is not associated to the maximum capacity. The load balancer can scale up above the LCU value to meet traffic demand, however, the load balancer will not scale down below the LCU value regardless of the traffic volume.

https://docs.aws.amazon.com/elasticloadbalancing/latest/network/introduction.html#network-load-balancer-overview https://docs.aws.amazon.com/elasticloadbalancing/latest/network/capacity-unit-reservation.html