These are only the ones we have seen, that have appeared in the game. No-one knows how many there are in total; there has never been an official lore statement as to the exact sizes of the rival fleets. I'd assume there'd be at least a hundred for each side.
We also don't know what classes they are. One should assume that not all the capital ships in the fleets are "battlecruisers" and "interdictors". Just because FD are too lazy to implement them, does not mean they do not exist in lore. The word "battlecruiser", for example, implies the existence of "battleships" - slower, more heavily armoured versions, with more firepower. One could also surmise other classes: "carriers" (with ten times the number of fighters), "troopships" (with very little offensive or defensive weaponry that need to be protected by escorts), etc.
On the other hand, the Empire naming an Interdictor after the reigning emperor at the time (Hengist Duval) implies that they don't have anything bigger or more grandiose to honour the emperor's name with.