Do they take the average stat of all wolves, or do they only check one wolf? If they only check one wolf, is this wolf picked at random from the party, or does the stat check fall on the wolf with the highest stat?

If I want my wolves to successfully track a scent, do I have to raise stealth on all of them, or can I just have one wolf with a really high stealth? Same with discovering things during exploring; is it better to distribute luck, perception, etc. among all exploring wolves or give it all to a smaller number of wolves?

It checks all the wolves in your party.