Zubat evolves into Golbat at level 22. [1] X Research source

Cave of Origin Entrance Granite Cave ( Zubat Only ) Meteor Falls Entrance Seafloor Cavern Shoal Cave Victory Road

Your friendship score with the Zubat will carry over when it evolves, so you can get a head start on your Zubat by capturing it in a Luxury Ball and treating it well.

Gaining a level. Assigning a Soothe Ball for them to hold. Giving them vitamins. Walking 256 steps with them in your party. Using Pomeg Berry, Kelpsy Berry, Qualot Berry, Hondew Berry, Grepa Berry, and Tamato Berry. Letting your Pokémon faint or using herbs, powders, and roots will all lower your happiness score. [2] X Research source

This requires a friendship score of 220 or higher, though you can’t check this directly.