Roman Imperial Navy Squadrons

The two Praetorian Fleets in Italy, the Classis Misenensis and the Classis Ravennas appear to have had 250 vessels each.
Small Squadrons in the Provinces seem to have consisted of between 10, 20, 40, and 60 vessels.

The smallest unit of river squadrons appear to have been based on 10 or 20 vessels.

40 Warships was the size of the Fleet at Trabzon, on the Black Sea. It was described by Josephus as ruling the Black Sea, along with 3,000 legionaries.
40 Lusoriae was the size of the fleet based at Mainz, Germania Superior under the Emperor Julian (361-363 CE), according to Ammianus Marcellinus (Roman Antiquities, Book XVIII, chap 2, line 12).

‘Sexaginta Prista’: ‘The City of 60 Ships’. This was the Roman Fortress of ‘Sexaginta Prista’ in Ruse, on the Danube in Bulgaria. The name meant the city of 60 Liburnians with rams.
60 Warships was the size of the Fleet in which Pompey set sail to eliminate the Pirates of Cilicia.
60 Warships was the size of Cleopatra’s Squadron of the Egyptian Navy, which was commanded by Mark Anthony, when she escaped from the Battle of Actium.

220 vessels seems to be a recurring number in the Navies of Grand Fleets:
220 Triremes is the figure Appian gives as the capacity of the ‘Cothon’, the naval harbour of the Carthaginian Navy in Carthage.
A Fleet of 220 Liburnians belonged to the Pirates of Liburnia. They controlled the Adriatic from their many islands off the coast of Croatia, and had invented the Liburnian Warship with its two banks of oars. They usually hired out their Fleet to the Kings of Macedon.

The Classis Misensis appears to have had a capacity of 250 vessels, mostly Triremes, and 10,000 sailors.
The harbour of the Classis Ravennas could accommodate 250 ships, according to Jordanes (writing in the 6th century), who was quoting Dio Cassius.

Most Roman Fleets were based in the Mediterranean or along the Rhine or Danube rivers and their tributaries.