Does joining a small instance affect your reach?

Will less people see your posts if you self-host your own instance?

No and no.

On my small single-user instance (atomicpoet.org), I sent this post 2 hours ago. As you can see, it now has:

* 784 boosts
* 354 favorites
* Too many replies for me to count

As I've now shown, size of an instance is no obstacle to spreading your message!