## Update ##
At least 5, and probably more
The in-universe journal "Rey's Survival Guide" offers the following information
Imperial Star Destroyers: These huge ships were the main battleships
of the Empire and most were torn to pieces by the time they came down
on Jakku. The ones that landed top-down are pretty much demolished,
but you'll find others sitting pretty much upright on the sand.
By definition, there need to be at least two that landed top-down (in order to be referred to in terms of multiples) and at least three other (since we already know of two).
Within the main canon (books, films, games, comics) there's no mention of any other Star Destroyers on the surface of Jakku. The 'Journey to the Force Awakens: Lost Stars' book makes a clear reference to there being a fleet in orbit of Jakku (and in an earlier passage vaguely indicates that there are more than two Star Destroyers in said fleet) but the only crashed ships we see on the surface, or hear about on the surface are the two SD's that you've already mentioned in your original question; The Inflictor and the Ravager
“We’re headed toward a major standoff with the rebels,” he said.
“We’re committing a fair portion of the fleet, and if the damned
Rebellion wants to stand a chance of keeping that sector, they’ll have
to do the same. This promises to be the largest battle since Endor.”
His long finger jabbed at the controls, and an image of a planet in
brown, russet, and gold hovered above the holo-projector. “Here we
have the desert world of Jakku—worthless on its own but soon to live
forever in history as the place where the Empire defeated the
Rebellion once and for all.”
Star Wars: Lost Stars
The fates of the other (unnnamed, and possibly not even present) Star Destroyers at the Battle of Jakku is an unknown.
Presumably they were destroyed in orbit or fled when it became clear that their command ships were lost.