In Speaker for the Dead there's this paragraph indicating that Valentine gave birth to five children during Ender's voyage from Trondheim to Lusitania.
The years passed, the family prospered, and Valentine’s pain at Ender’s loss became pride in him and finally a powerful anticipation. [...] It was Plikt, the good Lutheran, who taught Valentine to conceive of Ender’s life in religious terms; the powerful stability of her family life and the miracle of each of her five children combined to instill in her the emotions, if not the doctrines, of faith.
It's clear from the surrounding text that the children were Valentine's, not Plikt's.
In Xenocide the paragraphs describing the ship's accommodations during Valentine's voyage from Trondheim to Lusitania indicate that Valentine had three children, and mentioned them by name.
She was still imagining how a researcher would go about writing a grant proposal for such a project when she got to the four-bunk compartment they shared with Syfte and her husband, Lars, who had proposed to her only a few days before they left, as soon as he realized that Syfte really meant to leave Trondheim. [...]
Their twenty-year-old daughter, Ro, and Varsam, their sixteen-year-old son, shared another compartment with Plikt, who had been their lifelong tutor and dearest family friend. The members of the yacht’s staff and crew who had chosen to make this voyage with them—it would have been wrong to dismiss them all and strand them on Trondheim—used the other two. The bridge, the dining room, the galley, the salon, the sleeping compartments—all were filled with people doing their best not to let their annoyance at the close quarters get out of hand.
If there were other children, surely they would have been mentioned, even if only in passing, before mentioning the nameless yacht crew. Later there's also this exchange between Valentine and Miro:
MIRO: Andrew said you were the best person he ever knew.
VALENTINE: He reached that conclusion before he saw me raise three barbarian children to adulthood. I understand your mother has six.
A few hundred pages later in Xenocide, Valentine is depicted as having four children, but one of them, Lars, is clearly the husband of Syfte and thus a son-in-law, not a son.
The real researchers couldn’t spare time to take a shift as the inside worker, wearing the suit and just sitting there, watching him, talking to him. Only people like Miro, and Jakt’s and Valentine’s children—Syfte, Lars, Ro, Varsam—and the strange quiet woman Plikt; people who had no other urgent duties to attend to, who were patient enough to endure the waiting and young enough to handle their duties with precision—only such people were given shifts.
So the answer from the books seems to be five children in Speaker of the Dead and three in Xenocide.