How many seeds does it normally take to do Snow White?  Does it take 20 seeds?  That's my guess.  Thanks in advance.

If you use superfood, and have two vases so you can get two snowdrops from one plate of apple, you can (if you're lucky) do it with around 12 - 14 seeds.  It will take more if you don't get all of your dwarves on one plate of food each but you should do it comfortably with 20.

To follow this up, with double organic the amount of seeds would be around what number?

With double organics you probably need to plant twice for each colour of gems because of the pair of magies which can't be detered (especially with the gems for the rarer dwarfs) and more apples needed to get enough snowwhites for snowdrops.  I'm guessing 25-30 seeds if you only want to use double organics.

  I did a part of the fairytale challenge each time there was a free superfood weekend and in the week after christmas (tree gift). It took me a looooong time to get that gnome table (December 2016) and I often had one or two toadstools taking up space so I was good to the next free superfood weekend.

If you have more than enough seeds using double organics is no problem. But if you are a bit low on fairytale seeds like I was, superfood is the way to go.

I can't buy gold so I never have superfood that's why im asking. How often are superfood weekends?

It varies, can be 4-6 months inbetween

There's no set schedule for free superfood - it doesn't happen very often at all.

There were only 1 free superfood weekend last year, plus the week of superfood from Christmas, so not very often at all