Yes, there can be the problem of having too many cooks in the kitchen for one metaphor that may apply. It could also be quite costly if they are all expecting high salaries. Note that this is just verifying the existence of a bad case and says nothing about its probability.
Optimal depends on a number of variables that you aren't disclosing. What metric do you want to use realizing that there is a good chance of some gaming the system here. Similarly, what constraints do you have within your world that may make it different than what Google or Microsoft may have in contrast.
Too many senior developers can have the problem of either no conventions or too many conventions I'd think. While some senior developers may be good at adapting, if none of them are likely to introduce a convention, where would a team start? Conversely, there may be some senior developers that are die hard fans of some conventions that may require some conflict resolution to resolve.