How does an AWS Lambda function scale inside a VPC subnet?

Question

I understand the AWS Lambda is a serverless concept wherein a piece of code can be triggered on some event.
I want to understand how does the Lambda handle scaling?
For eg. if my Lambda function sits inside a VPC subnet as it wants to access VPC resources, and that the subnet has a CIDR of 192.168.1.0/24 , which would result in 251 available IPs after subtracting the AWS reserved 5 IPs

Would that mean if my AWS Lambda function gets 252 invocations at the exact same time,
Only 251 of the requests would be served and 1 would either timeout or will get executed once one of the 252 functions completes execution?
Does the Subnet size matter for the AWS Lambda scaling?

I am following this reference doc which mentions concurrent execution limits per region,
Can I assume that irrespective of whether an AWS Lambda function is No VPC or if it's inside a VPC subnet, it will scale as per mentioned limits in the doc?

Answer 1

Vladyslav's answer is still technically correct (Subnet size does matter), but things have changed significantly since it was written and subnet size is much less of a consideration. See aws' announcement :

• Because the network interfaces are shared across execution environments, typically only a handful of network interfaces are required per function. Every unique security group:subnet combination across functions in your account requires a distinct network interface. If a combination is shared across multiple functions in your account, we reuse the same network interface across functions.
• Your function scaling is no longer directly tied to the number of network interfaces and Hyperplane ENIs can scale to support large numbers of concurrent function executions

Answer 2

Yes, you are right. Subnet size definitely does matter, you have to be careful with your CIDR blocks. With that one last invocation (252nd), it depends on the way your lambda is invoked: synchronously (eg API Gateway) or asynchronously (eg SQS). If it is called synchronously, it'll be just throttled and your API will respond with 429 HTTP status, which stands for "too many requests". If it is asynchronous, it'll be throttled and will be retried within a six hour period window. More detailed description you can find on this page .

Also I recently published a post in my blog, which is related to your question. You may find it useful.