This is a course to learn Polish and you have to spell out the numbers to learn how to write and read. In Polish for English speakers you may get leniency on spelling in English, but not in Polish.
I just asked an Am. native speaker and "I am seeing" implies a long sequence of meeting 21 people in progress and has some sexual undertones, unless it is a doctor who says "I am seeing 21 patients today." I can also say "I am seeing 4 doctors today," but it has a meaning of meeting/dating, not the ability to see.