Single vs Multiple Dataset

This one comes down to governance and such. Many people start out with 1 dataset = 1 report = 1 dashboard = 1 app. As you are probably seeing, this can lead to quite a few inefficiencies, especially if many reports are really use the same source systems and data under the hood. If you leave this state of affairs go for very long, you will have a massive pile of spaghetti at some point that you will need to go in and clean up. 

It is better to get a handle on it sooner rather than later. Often, it is far better to create a kind of "golden" dataset that has been professionally modeled and cleaned up and prepared for self-service. Create this dataset without any reports. Then use this dataset via a Live connection or in the Service to create reports. More efficient, one place to maintain the data/data model versus 50. One refresh versus 50, etc.

It is also often a preference to go the Dataflows route to create reuseable queries that other people can use. Again, more efficient. Instead of the same query hiding in 50 different reports, 1 query that everyone uses.

Anyway, don't want to get on my high horse regarding governance but I recommend that you pay attention to it and choose a model that is right for your business. There is no single answer that is 100% "right" all of the time with this stuff.