If you search the github page for the project, many have reported using a raspberry pi.

For example, here is an issue where a raspberry pi 4 is used. https://github.com/immich-app/immich/issues/611

I used it on a Pi 4 for a month or two. Everything else was fine but the pi was really starting to struggle running the machine learning container, especially when I imported a bunch of photos at once. I have it on a beefier x86 machine now and things have been smooth.

I guess thats only an issue when you import photos though. Once they're indexed, it should run fine.