In this two entries in Eratta
In Chapter 59, Catelyn IX, it is said that Catelyn is welcomed by Lord Walder Freyin his hall while he is surrounded by "twenty living sons (minus Ser Perwyn, who would have made twenty-one), thirty-six grandsons, nineteen great-grandsons, and numerous daughters, granddaughters, bastards, and grandbastards". While Walder Frey indeed has twenty-one sons,[N 1] he has thirty-one trueborn grandsons[N 2] and seventeen trueborn great-grandsons[N 3] at the time.[N 4] Furthermore, the listing from the Appendix from A Clash of Kings makes it likely that not all of them are present at the Twins. Although the locations of each of these characters at the time of Catelyn IX are not confirmed, by the start of A Clash of Kingsthey are the following: listed as serving as a page are Willem Frey (at Ashemark), Emmon's son Walder (at Casterly Rock),Robert Brax (at Casterly Rock), and Wendel Frey (at Seagard); Further listed as being away from the Twins are Zachery Frey, being trained at the Sept of Oldtown,Luceon, in service as a septon at the Great Sept of Baelor in King's Landing, Robert Frey, in training at the Citadel in Oldtown,Willamen, in service as a maester atLongbow Hall, Bradamar Frey, being fostered in Braavos, and Malwyn Frey, being apprenticed to an alchemist in Lys. Additionally, Walder's son Geremy is said to have drowned, while Aenys's son Aegonis identified as an outlaw.
Here replace this:
Emmon's son Walder (at Casterly Rock),
Emmon and his sons and grandsons (at Casterly Rock).
In Note1 replace Geremy with Elmar
Add after this:
While Walder Frey indeed has twenty-one sons,
After Geremy's death,
Additionally, Walder's son Geremy is said to have drowned
Replace number of grandsons from 31 to 29( I guess at the time means during that chapter)
2)Ser Ryman Frey refers to his grandfather, Lord Walder Frey, as his father when Robb Stark arrives at the Twins for Edmure Tully's wedding. However, this might not necessarily be a mistake, as Robb refers to Walder as Ryman's father as well, while Ser Walder Rivers, Walder's bastard son, refers to him as his "father" (correctly) and "grandfather" (incorrectly) in the same conversation.
There is also a posibility, given the fact that Ryman's father is death, this was hidden threat to Robb.