"Mananaat traayate iti mantrah"
(That which upliftsby constant repetition is a Mantra.)
"shlokas" although they are often found in single line or Mantras were originally conceived in the Vedas. Most mantras followthe written pattern of two line even singleword form.
Shakti app have lots of Sacred Chants, "Meditation tips", "why need Mantras" , so you can remember "Mantra" easily. You can also set No. of times you want to "Jaap" that Mantra.