Wizards in 5e gain 2 spells at each level up -- twice the amount of other arcane casters (bard and sorcerer). This gives them much greater flexibility in what they can do and what roles in the party they can fill.
The PHB entry for Wizards has a sidebar titled "Your Spellbook," which contains the following text:
...You might find other spells during your adventures. You could discover a spell recorded on a scroll in an evil wizard’s chest, for example, or in a dusty tome in an ancient library.
Copying a Spell into the Book: When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it. Copying that spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it.
I understand that in most cases, this is going to be largely DM dependent, especially for home (non-published) campaigns. But, there must an average expected amount of loot for a published adventure (Curse of Strahd notwithstanding -- a bad adventure for wizards who hope to discover a few additional spells along the way!).
So, based on the currently available published adventures and AL modules, what is the number of "additional" spells a wizard can expect to encounter throughout a given adventure, say, per level gained?