As we tarried (epimenontōn hēmōn). Genitive absolute. Note epi (additional) with menō as in Act 12:16.
Many days (hēmeras pleious). More days (than we expected), accusative of time.
A certain prophet named Agabus (prophētēs onomati Agabos). A prophet like the daughters of Philip, mentioned already in connection with the famine predicted by him (Act 11:28), but apparently not a man of prominence like Barnabas, and so no allusion to that former prophecy.