I received only one bonus mystery skin when I bought a total of 24 Mystery Chests. Your website said that if you buy 6 you get a bonus one. I got Angler Jax after i won a game, but since I bought 24 skins using mystery gift I expect three more. That is how it was written.
EndlessSorcerer3 votesWe still have a couple days until they finish distributing the skins.
It's possible that they are broken up individually and that they will get to the rest of yours later.
Asher Dynamo1 votesOh, I sort of figured they would do them all at once. But yeah, i guess if they want to make it more exciting, its okay. But they weren't very specific in their statement. So it has me worried a bit.
Humble Fungus1 votesYea, same thing has happened to me... I bought the 5 mystery skins every time they came off CD, so I should be getting a LOT since it came off CD every 12 hours..that's 10 a day for like 7-8 days.. I should be getting a lot more than 1 skin. :/
ChiliPants1 votessame...just one 
skin, bought off CD 3 or 4 days
Humble Fungus1 votesI just opened a ticket regarding this a few minutes ago. I'll let you know what they say when I get a response.
ChiliPants1 votesthanks, definitely post back here bc still haven't gotten more than one. and its weird because nothing from riot yet on it
Humble Fungus1 votesI haven't received a response yet, but I did receive a 2nd skin a few minutes ago. I'm guessing that they're unlocking the skins when you would've bought that 6th skin.. so in another 12 hours I'll probably receive the 3rd bonus skin. That's my guess at least..
Humble Fungus1 votesOk, just be patient. I've now received a bunch of my skins. In addition to that, Riot responded to my ticket to let me know that I still have a couple more coming. It's just taking their system a little while to progress through the list. ;)
Darkanrath1 votesSame here. Got one for Olaf but I should be getting a total of 4-ish.
Darkanrath1 votesLets hope. I havn't gotten any extra yet, did most of my mystery purchasing a day or 2 after my first 6.