I'm designing a 3D-printed box/case for my Bela Mini. Do I need to think about a fan? or some air holes?

No need for a fan or air holes in the case however unless you need something waterproof it's probably better practice to let some air pass through. The board will heat up when in use and making sure it doesn't get too hot for too long will extend the life of the components. Many people have installed Bela and Bela Mini in waterproof and air tight containers for outdoor installations with no problems.