IDK, but I'm guessing it doesn't happen often. The argument against victim impact statements is that they're prejudicial against the defendant. In some cases where a judge felt a victim's statement was overly prejudicial against the defendant, they've ruled to have the jury disregard it. IDK. There's some discussion of it here:
https://scholarlycommons.law.wlu.edu/cgi/viewcontent.cgi?article=1420&context=wlucdj
Because not all of the Wags have filed motions regarding victim impact evidence, it could indicate a difference in how those defendants and their attorneys are approaching the case. It's possible (JMO only) that the defendants who filed those motions fully expect to fight their case to the finish, without a plea bargain, and want to have maximum advantage when it comes time for the jury to rule on their guilt.
If the Wags can convince the judge to eliminate victim impact evidence before the jury rules on guilt, they think they can get a "not guilty" verdict. JMO, they have this big strategy to use PR in the media, etc. to paint themselves as pious, law abiding, religious people and the Rhodens as dangerous, drug dealing criminals who everyone feared and hated and who deserved to die. Who knows, the Wags may even try to play this as some twisted version of "self defense" that they had to kill the Rhodens to protect themselves and S. If they can succeed in stopping victim impact statements, they think it will help their case.
JMO, the Wags also plan to paint all the surviving Rhoden, Gilley and Manley family members in an evil light, too. They will try to disparage their characters and treat them as dangerous, drug addicted criminals who would have been dangerous for S to be around. I'm guessing that's the argument they used early on after the murders, when they were successful in getting PCSO and DeWine to place the surviving children with foster families instead of with immediate family members. Reader said it was to protect the children from the potential killers, but at that point Reader and DeWine probably thought the family were the killers and that the family members were a danger to the children. JMO
Wags were successful in lying about the Rhodens and their families, painting them as dangerous demons, for a very long time. It took LE over a year before they finally started questioning everything they were told by the W's. Wags probably think they can pull this same game on a jury, too. That's why they don't want victim impact statements - they don't want the victims to be humanized. They don't want the jury to feel sympathy towards the Rhoden victims and family members by hearing them speak in person, express their grief and talk about the victims in a loving way.
It's much easier for the Wags to demonize the Rhodens if they're never allowed to speak for themselves, in person.
ETA: Thinking about the "all the lies they told us" statement about the Wagners from Reader at the time of the arrests, it would be interesting to know how soon FW began piling on the propaganda to protect her family of killers. Would it be possible to go back and see how soon she began telling lies to LE and the nature of those lies? Could examination of her "tips", etc. to LE reveal that she may have known about her family committing the murders right after it happened?