Sorry if this is a stupid question, but I'm super new to songwriting. How do you decide what instruments your song(s) need?

I've written a couple of songs just with my ukulele or guitar but I'm struggling to decide whether I need to add more instruments (such as drums, which I'd need to outsource), and how complicated they should be. I tend to write more indie-style songs, and get a lot of inspiration from the indie genre.