I came across this term in some texts dealing with the 1781 siege of Yorktown, Virginia. Many sources listed Admiral de Grasse's fleet as having 28 ships of the line (it wasn't hyphenated though...?), and that of his British counterpart, Thomas Graves, having only 14. These both sound like pretty big fleets, when you try to imagine them occupying the same small spot of ocean at the same time.
Then I found a text that explained that naval fleets were routinely described according to the number of ships of the line, but those were only the biggest warships in a fleet. The number (28 or 14) did not count all the transports and supply ships and what have you. Counting those vessels, de Grasse's fleet must have been closer to 100 ships. Hard to picture in the mind's eye.
The 28 ships of the line carried 2,078 guns and 18,138 crew members.
De Grasse's flagship, La Ville de Paris, carried 104 guns, and 1,165 officers and men. That's a floating city, is what that is.
EDIT: I just looked it up again. De Grasse's fleet included 28 ships of the line, and 47 other vessels (total), including: 14 "vaisseaux" 10 frigates 4 corvettes 2 cutters And presumably 17 ships of indeterminate type. (Supplies?) "Vaisseaux" (I don't speak French) translates to "vessels," but presumably these are warships of some type, or they wouldn't be enumerated separately.