What is the wizard proficiency for the Spell Save DC, is it the armour proficiency or what.
Each class gives one stat called proficiency bonus. It starts at 2 and increases from there. It is added to a lot of things like attack rolls, skills you are proficient with and your spell save DC*.
For example see the wizard class here.
The spell save DC is 8+ proficiency bonus +casting stat modifier (int for the wizard)
*Among other things