SAP PI 7.1 HTTP Adapter

In my scenario, SAP PI 7.1. needs to talk to a 3rd party EDI system via web services. SAP PI is using HTTP adapter to exchange XML messages for both inbound and outbound with the EDI system. Questions:

1. If SAPI PI sends for example 100 (different) messages to the EDI system every day, will PI make 100 web service calls to the EDI system to send each individual message?

2. Please explain how PI will use the HTTP URL to send the messages to the EDI system. Any document/blogs to explain how HTTP adapter works for exchanging messages with third party systems (web service capable)? Did not find anything good one in SDN.

3. Any disadvantage of using HTTP adapter, e.g. for high volume scenarios?