From TactWiki

Your Diplomacy Level is the main indicator of your progress in the sphere of Diplomacy.

You can view it at any time on the Equipment tab of the Diplomacy Equipment page on your character sheet. (Press C in game).

Just like the Adventuring and Crafting spheres, your Diplomacy Level ranges from 1 to 50.

Increasing your Diplomacy Level has a number of benefits:

• Some Quests require you to be over a certain minimum level to start the quest;
• Every 5 levels you gain an extra card slot in your deck whilst engaging in Parleys up to a maximum of 16 (when you reach level 45);
• You can engage in Civic Diplomacy Parleys with higher level NPCs.

To increase your Diplomacy Level you must carry out Diplomacy Quests or engage in Civic Diplomacy