寥寥 rarely stands alone — it lives in fixed pairings: 寥寥无几 ('so few as to be barely countable'), 寥寥数语 ('a few brief words'), 寥寥可数 ('few enough to count'). Learn it through the chunks.
register
Literary register. In speech you'd say 很少 ('very few') or 没几个 ('not many'). 寥寥 belongs to written Chinese — newspapers, essays, formal reports.
