Halo: Campaign Evolved brings back 26 Skulls from earlier Halo games.
These returning modifiers are commonly described as legacy Skulls. They draw from effects associated with previous Halo releases, including the broader Skull tradition that began with Halo 2 and later appeared in versions of Halo: Combat Evolved such as Halo: Combat Evolved Anniversary and The Master Chief Collection.
The remaining 16 Skulls are new to Campaign Evolved, giving the remake 42 available Skulls in total. The returning group therefore makes up most, but not all, of the Skull roster. That is why 42 is the total answer, while 26 is the answer to the narrower question about earlier games.
Another possible confusion comes from the 39 collectible Skulls found in missions. Three more unlock through Campaign Remix, but those availability mechanics do not change the legacy count: 26 of the game’s 42 Skulls return from earlier Halo content.