Are the rewards from scanning Amiibos a one time deal or can you scan more than once? Tried googling it but all that comes up is what rewards the Amiibos give, and not the frequency.
As said by Ari you can scan Amiibo's once per day but as mentioned by Timmy some things are going to be different.
For wolf link for example, you can scan him multiples time a day, as he will 'go back to his world' after a cutscene, entering a dungeon ... But you will be able to make him spawn again by re scanning the Amiibo. Except when he dies, you will then have to wait a day.
All the Zelda's Amiibos will give you some rare stuff, so most of the time you will gain meat/fish/plants depending on which amiibo you use, but sometimes a rare bow/sword will drop too. (see here for which amiibos drop what)
And finally there are some you really want to think about it twice before using as what they invoke will pop up only once and depending of where you are at the moment will possibly lost forever. But I'm not going to spoil you furthermore if you want to discover what will be invoked depending of which Amiibo and all :)
According to this IGN article:
Amiibo can be scanned once per day.
This implies you can also get the Amiibo's associated rewards once each day.
Here are some interesting notes from my personal experience.
If you have duplicates of any amiibo they are treated as individuals when used the day of. For example I can scan both smash bros link I have for extra stuff.
The BOTW specific amiibo definitely drop chests and good items AS WELL as any other Zelda related amiibo!
Using any other amiibo not related to Zelda is still worth it as they drop bout 6 items or so (random things too).
All the meat you get from amiibos is also great to sell for a quick buck.
In addition to Vemonous' answer, one cool thing that I noticed is that duplicate amiibos appear to be totally unconcerned with special items. While it's kind of useless having two Eponas, that's totally a thing I have going on now and it's pretty rad.