
You have an 87% chance of getting a legendary with 20 shards during this event.

If you pull 20 sacreds during a 2x, you have a 97% chance of getting at least 1 legendary, a 71% chance of getting 2 or more, and a 44% chance of getting 3 or more. You have a 65% chance of getting a legendary with 15 shards during this event. If you pull 15 sacreds during a 2x, you have a 87% chance of getting at least 1 legendary, a 55% chance of getting 2 or more, and a 27% chance of getting 3 or more. You have a 46% chance of getting a legendary with 10 shards during this event. If you pull 10 sacreds during a 2x, you have a 72% chance of getting at least 1 legendary, a 34% chance of getting 2 or more, and a 11% chance of getting 3 or more. But looking at some more realistic totals: Now looking at sacreds, I'm not really going to consider the pathological case that have to pull 59 sacreds to get a legendary because it's so unlikely, but FWIW if you pulled the same 59 shards during a 2x, you'd have a 99.5% chance to get at least 2 legendaries and a 73% chance to get 6 or more legendaries. So on average, you're better off pulling during an extra legendary. If you pulled the same 220 shards during a 2x, you'd have a 65% chance to pull 2 or more legendaries (although that does include a 38% chance you'll pull more than 2).

Let's say during an extra legendary from ancients your mercy counter is at 0 and you get really unlucky and have to pull 220 shards to get a legendary.

The math comparing this to 2x is a little more complicated due to the way that mercy fits in, but let's look at a few scenarios, starting with looking at ancients. TLDR: Because the base chance of pulling a legendary from sacreds is so much higher than ancients, pulling during this isn't nearly as good an idea as it is for ancients.
