Erdós is described in Hoffman's biography ('The Man who Loved only Numbers') - I can't recall, nor immediately find by whom - as only being able to count up three numbers: '1, 2, n'.
Reminds me of the "zero one infinity rule"[0] rule for programming. Roughly stated it says that software should be designed around 3 cases: 0/no instance of a situation, 1/single instance, or an infinite/limitless instances.
