What does the number "21" on this Rekall ad stand for?
That's not a 21, that's another language. It's a Korean transliteration of the name 'Rekall' so people who read Hangul (the written Korean language) and not English can understand the ad. Fortunately, Hangul is relatively easy to read, and is well-documented online. After some googling, I found a few tutorials, and a handy Wikipedia reference. Hangul has each 'letter' be a syllable, which combine more fundamental vowel and consonant symbols. Here that means we have 2 syllables - "Re" and "kall":
Therefore it's "re"->리 + "kall"->콜 = 리콜