Please use the GetRecordedAtTimeAdHoc method from PI Web API. Using this method, you can specify multiple streams with multiple instances of the parameter.
Concerning the WebId, you must make an HTTP call for each WebId you want to get. Nevertheless, if you use PI Web API Batch, you can make a lot of requests in a single requests.
There are plenty of material on PI Square about this topic:
Please let me know if you still have any question,