To determine whether you need an Enterprise Service Bus then we need to look at the overall SOA architecture.
Below are the few common components that most of the web sites will provide.
Adapter: Application module added to an application internally or externally to allow access to its functionalities via a standards-compliant services interface.
Business Process Modeling: Procedures for mapping out the business processes do both in terms of what applications are expected to do and what the human participants in the business process should do.
Service Broker: SOA framework that brings Web Services together using the rules associated with each component and make available to public requester.
SOA Governance: SOA administration states administration quality, consistency, consistency and execution, guarantees that work force take after endorsed arrangements. It ensures that owners follow prescribed policies and corrects system problems or policy infractions as they occur.
SOA Registry: A single source to track all the metadata and publish web services to applications developers in a SOA environment.
Enterprise Service Bus: As Wikipedia describe “The concept is analogous to the bus concept found in computer hardware architecture combined with the modular and concurrent design of high-performance computer operating systems. The motivation for the development of ESB was to find a standard, structured, and general purpose concept for describing implementation of loosely coupled software components (called services) that are expected to be independently deployed, running, heterogeneous, and disparate within a network.”
By now you should have an answer. ESB is good but do you have heterogeneous web services to integrate. Do you need a bus to carrying multiple passengers (web services) to drop off at different bus stations (.NET, Java, BPEL). If 70% of your integrations are point to point then you only need a car not a bus. Of course, no one can stop you from buying a bus to carry one passenger.
Another point to think is do you have anything to decouple at all. For example, all the 20 business functions in your organization need to go through a known system then basically your chance to decouple the business functions to different components could be none. Some more what is the reason of doing that.
Microsoft ESB solution
If you are looking for a Microsoft ESB solution then it will be Microsoft Biztalk.
Service-Oriented Architecture Components (n.d.). Retrieved 22 October 2017, from http://www.dummies.com/software/service-oriented-architecture-components/
Service-oriented architecture. (n.d.). Retrieved 22 October 2017, from https://en.wikipedia.org/wiki/Service-oriented_architecture
SOA governance. (n.d.). Retrieved 22 October 2017, from http://searchmicroservices.techtarget.com/definition/SOA-governance
Enterprise service bus. (n.d.). Retrieved October 2017, from, https://en.wikipedia.org/wiki/Enterprise_service_bus