I would rephrase as N customers, where N is enough to earn some significant percentage of your costs. For many kinds of businesses N might be only a handful, it

I would rephrase as N customers, where N is enough to earn some significant percentage of your costs. For many kinds of businesses N might be only a handful, it completely depends on your market. For others 30 is nowhere near enough I'd imagine. Of course, determining significance is left as an exercise for the reader.