Genshin Impact: Sprayfeather Gill Route & Farming Guide

There are a total of 73 Sprayfeather Gill in Natlan, not including the one from the shop (if there is any). Keep in mind that some of the Sprayfeather Gills are located inside a locked area and you need to complete certain quests to be able to unlock it.
In this guide, there will be a link to the official interactive map. So you can mark your pins (to see which you have already collected them) and follow along the route.

Teyvat Interactive Roadmap

28 Sprayfeather Gill in North of Ameyalco Waters

15 Sprayfeather Gill in South of Ameyalco Waters

13 Sprayfeather Gill in Southeast of "People of the Springs"

12 Sprayfeather Gill in The Upper Sanctum

Important: The local specialty in this area is locked behind the quest chain [Tale of Dreams Plucked From Fire]. In this quest, you will unlock the Spiritway to The Upper Sanctum, and only then can you obtain these Sprayfeather Gills.

How to Start Tale of Dreams Plucked From Fire Quest

Teleport to the S7 near Ameyalco Waters and look south for the ruin with an opening so you can get to the underground area. Go inside there and keep walking until you see a young woman. Talk to her and you will start the quest - Tracer No Tracing (which is the first part of the Tale of Dreams Plucked From Fire).
You can see the full guide here: Tale of Dreams Plucked From Fire Quest Chain Guide

5 Sprayfeather Gill in the Underground Lava River: Lower Level

This isn't locked per se because if I remember correctly, you don't have to do the related quest but still able to enter the Underground Lava River. But to progress to the deeper part, you do need to start the quest [Tale of Dreams Plucked From Fire Quest Chain], which is the same quest as in the previous route.

But to get to the Sprayfeather Gill, it's hidden behind a simple puzzle (unrelated to the quest). Approach this seelie-looking creature (called Monetoo in natlan) and it will go back inside its mural. Infuse the mechanism with Phlogiston and the hidden door will open.

Inside this hidden cave is the 5 remaining Sprayfeather Gill.