Depends what you mean by relevant I guess.
Some parents are good parents. Some parents are ok. Others are downright feckless. The children of good parents can have multiple half-siblings and very little, if any, negative consequences from a complex family structure. However, bad parents or dysfunctional relationship dynamics can make that same family a really bad place for children to grow up in.
It would be naive to assume that there aren't special considerations for blended families. But they are in no way insurmountable and can, in some cases, actually be a plus factor.
The relevant bit is the standard of parenting and the relationships between all the parents involved, not the actual number of parents IMO.
Sadly, however, the more people involved, the more likelihood there is that one or more of those parents will be a bit of a twat who messes it up for everyone, but most especially the children.