Complex sentence: Only two of these families have a substantial number of species (Diapriidae and Proctotrupidae), which together contain over 2000 species, and the others are all tiny, often relictual groups.
Split into simpler sentences: Only two of these families (Diapriidae and Proctotrupidae) have a substantial number of species-together they contain over 2000 species. The others are all tiny, often relictual groups.