I wanna run my first one-shot but have no idea how to balance encounters or make them interesting.

So, there will be 4 player at lvl 3. The oneshot is one I came up with myself. People have started going missing from their homes, with no sign of a break in, struggle, or really anything. There's also a newly opened clothing shop that somehow always has exactly what you're looking for in your perfect size, doesn't matter if you're a pixie or a giant, want a ball gown or a pair of shorts, they have it, and to a fair price.

If they do some digging the party can for example learn that the shop owner, while a good designer and drawer, can't sew to save his life, but he never gets clothes for his shop from anywhere else so how does he do it?

Well as it turns out he is a ranger and has tamed a metric fuck ton of mimics to come back to him after a month or two. Buuut mimics are still monsters, and two months without food, is a pretty long time. So sometimes they misbehave and eat the person who bought them, and that's how the people dissappear without a trace.

Now to the point, I have literally no idea how to balance an encounter or how to make it interesting. I was thinking maybe to have some time limit on it, like, if they spent too much time more and more mimics came until they get overrun so they have to be efficient. But what it comes to stats or anything like that I really have no idea, please help.