Compounds of １つ

• １つ
  (n) (1) one
  (2) for one thing (often used in itemized lists)
  (3) (after a noun) only
  (4) (with a verb in negative form) (not) even
  (n-adv) (5) just (i.e. "just try it")
• ２つに１つ
  (exp) one of two (possibilities, alternatives, etc.)