Den of the Wolf says "Limit once per game". Since discarding is the cost I understand that this limit is per card title (just like events), not per effect, right? So any other limits (per phase etc.) should work the same unless I'm missing something.

You are correct that normally, a limit on a card effect triggered from play would apply only to itself unless it specifically says otherwise (for example, Limited Responses specifically say the limit applies to all "Limited Response" effect, whether you are triggering from play or not).

"Limit 1 per game" is an exception to this general rule, though. FFG has specifically said that "1 per game" limits apply to all cards in your deck with the same name, regardless of whether the effect is triggered from in-play or out. It is written down as part of the "Patchface" errata entry in the Legacy FAQ and was extended to all "Limit 1 per game" effects by Nate in a board ruling.