Recommended Lambda function size

The concurrency of the function is not related to the memory size of the function. The Lambda infrastructure will create multiple instances of your function to handle multiple requests, and each instance will have a seperate block of memory of the configured size.

If you're aiming for 50,000 though, you will have other problems, the first of which being the default concurrency limit in lambda being 1000 (https://docs.aws.amazon.com/lambda/latest/dg/limits.html).