I think the guest count matters, but it's not the whole story. You could have fifty people and still find the booth sits idle if your crowd isn't into that kind of thing. What actually drives usage is whether people at the party are comfortable being silly together and whether there's a natural gathering spot for it. A 30th birthday with close friends and family who know each other tends to work better than a mixed crowd of coworkers and distant relatives who don't mesh as well.
The printer part is honestly what seals the deal for me. Sure, people take phone photos constantly, but there's something about getting a physical print right then and there that makes it feel special. Your guests actually have a tangible keepsake, and they're more likely to gather around the booth specifically to get those prints. I've seen people spend way more time posing for something they'll actually take home than for content that just disappears into their phone. If you're at thirty or more guests and they're the type who'd enjoy group shots and silly props, it probably pays for itself in the experience alone.
One thing that could tip the scales: check if the rental company offers digital copies too, or if you can get unlimited prints instead of a limited package. Some places are flexible on pricing if you're just renting for a shorter window. You might also ask whether they handle setup and breakdown - that's worth paying for rather than wrestling with equipment yourself on party day.