Is there a name for letters that follow a number, such as the "st" in 1st or "nd" in 2nd?
This article calls them "ordinal numbers", whereas the "cardinal numbers" are 1, 2, 3, etc. It goes on to say that they're expressed in English with "letter suffixes". The article on ordinal indicators simply calls them suffixes also.
However, the article on English numerals calls them "ordinal units", though it also goes on to say that any number with the last digit of 2 will have the suffix "nd", such as twelvend, so I'm not sure if that lends credibility to that section.
Good question. I love the chance to look something up and learn a bit myself. My takeaway is that I'd just call them suffixes.