The USPTO has created a patent class for tax patents. The classification is 705/36T.
The USPTO has placed 209 issued US patents and 188 published patent applications in this classification. The USPTO has not, however, published a formal definition of the class.
About 10 new tax patent applications have been filed each year in recent years, and about five new patents have been issued each year. Some applications and issued patents appear to be mischaracterized since they do not deal with taxes.