The URL identifier (the part after farp.me/) is limited to a maximum of 4 characters in length. This limits the number of URLS I can have but it also ensures that the URLs that are generated are short and actually rememberable.
So each identifier can be 1, 2, 3, or 4 characters long. There are 62 possible characters (a-z, A-Z, 0-9). That means there are 62^4 + 62^3 + 62^2 + 62 possible URLs. In other words, when it gets to about 15,000,000 URLs my algorithm will need to start making longer URLs. If it ever gets that big, I’ll be shocked, SHOCKED.