Google stated that the new update in Google Search is to give users variety in search results, which means top results will not contain more than two results from the same domain.
However, it might not be the case every time, as Google has the right to display more than two results when it is required and results are more relevant. It would be the case mostly when the queries are related to brands specifically, like searching Amazon.
This site diversity change means that you usually won’t see more than two listings from the same site in our top results. However, we may still show more than two in cases where our systems determine it’s especially relevant to do so for a particular search….
— Google SearchLiaison (@searchliaison)
Sub-domain will also be considered part of the main domain and will be shown only the results of the main domain. Like blog.example.com will be one of the results of example.com.
Still, Google has the right to considered sub-domain separately when there is a need to show diverse yet relevant results.
This new update will only affect the core results of the Google, whereas, other search features like top stories, videos, images, and other searches will remain unaffected, said .
It would not impact the ranking of pages as it would remain the same. However, only the URLs shown will be changed in the search result.
The changes are not perfect, and Google might continue showing more than two results. But with time, it will be improved. Tweets also stated that this change is different from the diversity to be implemented from June 2019.
A site that tries to dominate the search results for a certain query may be impacted the most than any other areas of research.