Whatever I set this config value to, localstack anyway starts new lambdas on any new message in a queue no matter how many are already running.
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Hi @mif 
The MaximumConcurrency ScalingConfig for SQS in Lambda Event Source Mapping is not yet supported. However, we are actively working on it. Stay tuned for more updates in the coming weeks 
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As a temporary workaround, you can use Reserved Concurrency on the Lambda function itself.
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Hi @mif
Quick update: Concurrency is still not supported in our new implementation, but a lot of other things improved 
. We created a behavior coverage feature table to communicate it openly: Lambda | Docs
Feel free to create a GitHub issue if you are missing this feature.
Nevertheless, worth sharing: Check out our new Lambda event source mapping (ESM) implementation in LocalStack v4 offering better functional and non-functional support for ESM .