This is how the menu gets the number.
int32 FInternalPlayWorldCommandCallbacks::GetNumberOfClients()
{
const ULevelEditorPlaySettings* PlayInSettings = GetDefault<ULevelEditorPlaySettings>();
int32 PlayNumberOfClients(0);
PlayInSettings->GetPlayNumberOfClients(PlayNumberOfClients); // Ignore 'state' of option (handled externally)
return PlayNumberOfClients;
}