Is there a way we can speicfy Concurrency for lambda function through Serverless.yaml?
Nope, not yet.
If you want new functionality, the GitHub repo issues is the best place to request it - there’s already one for this functionality.
This made it to release 1.27.0: https://github.com/serverless/serverless/releases/tag/v1.27.0. See https://github.com/serverless/serverless/pull/4694 for details on use.