When I complete the feat for using mythstones, it says you get +2 stack size.
Does that mean I have to have a mythstone in my inventory to get +2 more? Or do you just get 2 of all of them?
all of your ms are increased by 2, if you have 100 gifts ms, when you complete, you will have 102 gifts
Never mind. It already rolled over. You get 2 of all the ones you own, which, is still really great 
Yeah what Ninja said is correct but in case it was still even slightly ambiguous it is ONLY the ones you have… So you want to combine up to the highest mythstone you can before completing the feat so you maximize your benefit (also try to have at least 1 of the highest 3-5 mythstones you have access to).
Edit: Yep, you beat me to it – make sure you keep track of when you’re going to complete the feat so you can convert up any mid mythstones you have to have the highest mythstone possible and at least 1 of as many “high” mythstones as you can.
Agree. I was 60 away when I did the first post. It snuck up on me, but I had quite a few singles that I now own 3 of. Planning my next roll over…
Just getting 1 of the highest MS/CS you can get by combining is beneficial. The +2 you receive for feat completion, makes it instantly possible for you to go an MS/CS higher.
Admittedly it might take several feat completions to to eventually work up to the highest MS/CS, but at least you know that at some point, with a little bit of work, you will be able to own the highest stones and gems available, and not have to rely on the RNG gods 
And not forgetting your earning some good hero points in the process 
Its really not even that hard to do… Say you have 500 of the fourth CS… Salvage 400, then salvage them again, and again. You just used 1200 stones, and can even convert them back up for 200 more and salvage back down for 200 again. 1600 uses. All it takes is gold, and even then, with that reduced to 10%, not a lot of gold. My cs and ms pools are growing. Farming gear is finally within my grasp! Woot!
Agreed
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In testing, there was no mass-convert, and converting/salvaging didn’t increase the counter for the feat… For crystals this meant having to manually use low crystals one-by-one on items (and remember you can’t use the Enchant crystal is the item is already full on affixes…) and was an incredible pain in the ass. Mythstones were even worse – you had to actually farm items, then go through manually and find the ones that had sockets to apply mythstones to. One by one, and with the bajillion clicks to apply it in addition to the time actually looking through the sea of blue trash for things with holes to jam them in. We also had to walk uphill in the snow both ways! WHILE USING ICE WEAPONS THAT WEREN’T EFFECTIVE IN THE SNOW!!!
So… It was harder?
That’s what she said…
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shakes head and sighs