Lemmy has two core contributors. It makes use of a service called pict-rs to allow image upload, which has one other core contributor.

If you can code in Rust, you can try contributing code. Otherwise you can report bugs in the issue tracker on GitHub. Or you can help with translations. Links to further instructions can be found here: https://join-lemmy.org/docs/en/contributing/contributing.html

Edit: Jerboa has one core developer as well but they are also one of the core contributors of Lemmy.