Amplifying Emblems are used to upgrade Signature Items from level 11 to level 20. 70 Emblems in total are needed to upgrade a Signature Item from level 10-15. 240 Emblems in total are needed to upgrade it from level 10-20.
Amplifying Emblems are less common than Primordial Emblems, and acquiring them can be quite expensive. They can be obtained from the currently ongoing Tavern Bargains event, higher Faction Tower floors, gift sets, special events, and AFK Rewards starting from Chapter 18. They can also be bought from the Store for Gold or Diamonds.