Ahhh, amiibo, amiibo. Those interactive figurines have become a commercial success. It's growth in popularity has resulted in  the production of a lot of them. So many, in fact, that some gamers are finding problems justifying their existence. For 12-13€ you get the figurine and all the content it unlocks, but how much is that worth? Some amiibo only work in a couple of titles, and some work on a lot of them. Chibi Robot amiibo only really works on one game, while FE amiibos work on S.T.E.A.M. FE Fates and Smash in major ways. Just by that, some amiibos are more valuable than others. And also, we have to take into consideration the games they are used into. Toad Treasure Tracker only really needs Toad to unlock all of the amiibo content (it's compatible with other amiibos, but they give you bonuses like extra lives), but for AC Home Designer you need a lot of amiibo cards to have all the amiibo content.

On how many games should an amiibo work to be worth purchasing? I'm not counting effects like 1-ups, extra damage or difficulty alterations, I'm talking about uses that give you actual unlockable goodies, like skins, levels, minigames or other big features.

It depends on the content of the amiibo I guess.

Marth is pretty valuable if only because of FE Fates and Codename Steam.

Meanwhile, while Mario is compatible with several games, the features the Mario amiibo provides isn't too substantial in most games, so I don't consider him that valuable or worth the cost.

I believe for Ültra the answer is one. Only Smash.

I collect the figurines. The number of games they apply to is completely inconsequential for me.

Hmm, are we talking about the games or amiibo themselves? If we're talking about games that have amiibo usage on them, I would say the majority of them, since most of them are already great games on their own, and amiibo are not needed to enjoy them. Only one I can think of is Mario Party 10, amiibo or no, that game doesn't seem very good. Amiibo Fest I would def say is not for everyone either.

I'm talking about the amiibos themselves. On how many different games should an amiibo function (significant use, not just optional like hearts and arrows, things more substantial like mods or skins) to be worth its purchase?

Zero.

Only one, the skylanders type game for amiibos that nintendo should made.

Honestly, all. I'm the type of person who doesn't collect or care about them. But I want the DLC. But then again. That's only $12 for who knows how much of actual DLC cost. Which is why it won't happen.

It currently heavily depends on each amiibo. Toad was until November used for a couple games and the usage was a waste of time. But Shovel Knight unlocks goddamned Co-op and a bunch of new abilities for that file that legitimately change the way you can play the game. For me it's quality over quantity there. Not that I really care, the gameplay features on these things are pretty much a bonus; they're designed as NFC chips solely because they knew it'd attract a slightly larger audience versus making them collector's items, and until the functionality is the primary reason for their design I'm not gonna be a child and moan about a bonus feature.