You can do as many concentrations as you like – or choose to do none at all.
In general, you should take 3 classes in an area to have a concentration in it. For Finance, you should take 4 classes.
Concentrations are not recorded on your transcript, or monitored in the same way that undergraduate majors are tracked. They exist only on your resume, and were developed to give students guidance on which classes they should take to become knowledgeable in certain areas of specialty.
In general, you should choose electives from the “strongly recommended electives” section to count toward your concentration. Treat the courses in the “recommended electives” as courses that complement your concentration studies, and are taken in addition to the courses you count towards your concentration.