Why Does Boruto and Himawari Have one less Whisker than naruto?
Naruto Uzumaki (3 Whiskers)
Boruto Uzumaki (2 Whiskers)
Himawari Uzumaki (2 Whiskers)
There is no guaranteed answer to that. But judging from various traits of different characters that they have acquired through parents, I can say that children of Naruto have inherited the trait of having whisker scars but not the number of scars.
It is evident from the creators of the manga giving part father and part mother traits to their children. Like in case of Naruto, he inherited his father's hair color although Kushina's hair had a distinctive red unlike other Uzumaki's. On the other hand, Naruto got his whiskers because of being exposed to Kurama's chakra while he was in his mother's womb because Kurama was sealed in Kushina.
Similarly, in case of Inojin (son of Sai and Ino), he inherited his father's face and his mother's style. In Boruto and Himawari's case, they got the whiskers because Naruto had and inherited the Jougan (Pure Eye - Boruto) and Byakugan(Himawari) from their mother.
This I how I feel Boruto got his whiskers. It also may be as simple as the fact being that Naruto and Boruto have been created by different people, and they might want to distinguish Boruto from Naruto.