It is additive. 

When the mercy system begins and you're effectively at a 0% bonus, so each time you don't pull adds 2%. It would go from 0%/2%/4%/etc. until you reset with a mercy pull. 

That was my hope.  

It would be nice if we could see a counter until Mercy is active.

This has been a common request since it was first put in, but Plarium are adamant as not implementing it.  There used to be third party programs that showed it, but the use of them quickly got you banned, which further shows that Plarium have no interest in showing this information unfortunately.

The reasoning is likely because it actually reduces the gambling nature of the game.  If people know how far they are in the Mercy process then they could hold off for certain events (e.g. 10x champions), and Plarium would prefer that you instead keep pulling

I am fairly certain that this is one of the features we will never see

Ya the only solution is to keep track manually. Keep a notepad and a pencil close by (ya those things still exist!) to count all the shards you pull, throw the paper out when you get some orange lightning :P

So I got my wife to play the game 1 month ago (right after Delianna FREE Lego) 

She has probably 8 lego (all from ancients)

She pulled 36 Sacred shards and yet to pull a Legend O.O

I know mercy for sacred does  not  kick in until like 50 sacred, but a new account going this  long is crazy to me

Just real bad RNG

RNG will turn in her favor and she might get 4 Legos out of 6 sacred shards.   Thats when the game is really fun.