It will likely be difficult for the parents to produce 6 children unless they both "definitely" want them, since parents who don't want a lot of kids will resist your efforts to have a large family. Even when both parents definitely want kids, you might have to be persistent, because their ability to produce children decreases with age and the number of children they've already had (baby boost can help with that). Unlike with the original VF, the couple's mutual desire for children has a significant influence on family size.