When talking about APIs on Lambda Function, Api Gateway is the most common subject. Lambda functions can be invoked by a handful of services such as SQS, SNS, Api Gateway (API and Websites), S3, IoT, Aurora RDS, etc. Due to it's low cost for execution and no hourly cost, Lambda with API Gateway makes a perfect match for low-demand services. But Api Gateway can be more expensive than a Load Balancer if you have enough requests.
If you want to know more about setting up a Load Balancer for multiple lambda functions, https://blog.deleu.dev/one-load-balancer-to-rule-them-all/.