My search was extremely disappointing.
Many sources would again not open up for Europeans, others had the critical pages excluded from Google books, and the rest mostly used the word "Fort" and "Fortress" as synonyms.
So my answer is: 1 fortress, named Fortress Monroe, because it is the only one I know that has the word "Fortress" in its official name. Although at the outbreak of the Civil War it had been long renamed Fort Monroe. But the "LoC Civil War desk reference" also connects the word "Fortress" only with Fort Monroe.
Other sources say presidios were fortresses, that would include the Presidio at San Francisco. Another source named Alcatraz a "military fortress", and again another source said Fort Ticonderoga was the significant fortress between the war of 1812 and the Civil War. There were more examples, naming other forts as fortresses, e.g. Fort Jefferson
As these sources seem to name Forts/Fortresses at random, I do not even list them here.
Congrats to those who found the right number of forts.