You decide how much structured data (whether with Schema.org or any other vocabulary) you provide, just like you also decide how much content you provide.
Just like there is no one that could dictate that your blog post must not cover more than 5 different topics, must not be longer than 10.000 words, or must not mention more than 50 persons, there is no one that could dictate that you must not provide more than 20 Schema.org entities, or must not use more than 50 properties.
In general, more good data is better than less good data, right? Consumers (whether humans or bots) can parse your documents and decide for themselves which of the structured data they are interested in, and ignore the rest. That’s the advantage right there: more structured data = possibly more/better use for consumers.
Disadvantages? None directly. Of course file sizes (and thereby, loading times) increase, and you have to invest more work (for implementation as well as maintenance), so you might want to do a cost–benefit analysis.
In the words of Schema.org
From their Getting Started documentation:
More is better, except for hidden text. In general, the more content you mark up, the better. However, as a general rule, you should mark up only the content that is visible to people who visit the web page and not content in hidden div's or other hidden page elements.
From their FAQ:
Q: Why should I add markup? What will I get out of it? How will the data be used?
Search engines are using on-page markup in a variety of ways. These projects help you to surface your content more clearly or more prominently in search results. Not every type of information in schema.org will be surfaced in search results — you can refer to each company's documentation to find specific uses — but over time you can expect that more data will be used in more ways. In addition, since the markup is publicly accessible from your web pages, other organizations may find interesting new ways to make use of it as well.
In the words of the consumer Google
From their Structured Data policies:
When you have multiple entity types on a page, we recommend you mark up all entities on that page to help Google algorithms better understand and index your content.
