Hello @Sureshkumar Nandakumar , apologies for the delayed response here. I am not very well versed with Azure DevOps but you can go through this documentation for cost implications of running parallel jobs. If you have any additional queries could you please redirect them to this Azure DevOps Forum. Please let us know if you have any concerns here.
Regarding alternate way to enable private end points for Azure resources without whitelisting Azure DevOps agent IPs. I think you should able to achieve that if you have a private IP connectivity established with the Azure resources from where you are running your agent pool. There are multiple connectivity services available in Azure Networking services which you can leverage.
As per the documentation
Please let us know if you have any additional concerns. Thank you!