Gen 5 is planned to be split, Gen 6 doesn't have enough to be split and I suspect 7, 8, 9 will be split however in which way will be interesting. The Ultra Beasts and Paradox Pokémon (related to legendarys) are supposed to get there own mini chapter meaning you find and breed but no capture (at least for the ultra beasts maybe not a rule for the paradox) so we could see 7 and 9 be split into three updates with the third being that mini chapter but we will see.