Vertex shader with many triangles

Hello!
I wanted to write my own shader. Read the text of the available shaders.
Everywhere the vertex shader "simple fullscreen triangle" is used.
My question. How can I use a vertex shader with a large number of vertices?
For example, I want to do post-processing of a part of the screen. For this, I need to specify this part with triangles. There is no such information in the documentation.
Can I give instructions?
Thank you in advance.
sorry for my English.

ReShade currently draws a fixed number of three vertices. I always wanted to make that configurable, but never got around actually implementing it.