The 12 million page mark was just met this month, so to celebrate I have embedded 3 YouTube training videos the I created so you can make best use of Chronicling America. Together there are about 25 minutes that you can watch.
There are additional resources that one should know about, besides searching.
The first resource to be aware of is the U.S. Newspaper Directory, which provides detailed information for over 154,000 newspaper publications and what libraries and archives hold them. Since only a small percentage of newspapers are in digital form, this is excellent information for the serious researcher. Below is a video that shows you how to find these offline collections and where to find the microfilms and original newsprint for those collections:
For the second resource, the video below shows you exactly how to find what collections are available for you to search online in the site, including collections by state, by ethnicity, and by language:
And yes, there are over 12 million pages that can be searched online. Below is a comprehensive video tutorial that shows you all the features and options that you need to know about in order to more easily and successfully navigate the site and search for articles about your ancestors:
There are newspapers from over 40 states and the collection is always growing. Some of the states that are represented have additional state and/or county-based collections not affiliated with the Chronicling America site, thus providing even more digitized newspapers to search. And there are also state and county-based digital collections for the states that are not represented in the national Chronicling America collection.
Please take a look at the Newspaper Research Links page in this, The Ancestor Hunt site for the aforementioned additional state resources as well as some special collections, such as German, Catholic, Jewish, Farming, and African-American newspapers.