Ive had the Thornfire Wolf spawn as little as twice and as many as four times when putting out fires in Shadowthorn during the World Change. During these times there has been no one else there. Because of this I make the assumption that every time you put out a fire it gives a small percentage chance that the Thornfire Wolf spawn occurs.
This would mean that there is no limit to how many times it could spawn. You could get very lucky and have it spawn more often, or very unlucky and not have it spawn at all.
This is just from my observation of course. I cant find any useful stats on their appearance/spawning
Hope this helps
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