We see the senators and the Duke in a very important council of war at the beginning of Act I scene 3. Although the precise number of ships is not specified, and indeed the group of leaders have received differing accounts, all reports agree that the fleet of galleys, however many they are, are a Turkish fleet, and that they are heading towards Cyprus. Note what the Second Senator says:
As in these cases, where the aim reports,
'Tis oft with difference--yet do they all confirm
A Turkish fleet, and bearing up to Cyprus.
Of course, news arrives to suggest that the fleet is going to attack Rhodes, which the group of leaders shrewdly interpret as a deflecting tactic on behalf of the fleet, as Rhodes was more heavily defended than Cyprus, and Cyprus was actually far more important to the Turks. Of course, the military meeting is interrupted by the arrival of the main characters and Brabantio's charges against Othello.