One backlink won't do much? That's the kind of blanket advice that keeps people stuck in the 6-figure rut.
A single, properly targeted backlink from a high-authority, relevant site can shift the needle-if everything else is tight. The issue isn't the number, it's the context. If you're chasing a bunch of weak, directory-spam links, then yeah, one won't matter. But a single link from a site that actually sends referral traffic and has topical relevance? That can tip a page from page 3 to page 1.
Most people treat backlinks like a volume game. They're wrong. One link from a real source is worth more than fifty from automated PBNs or guest post farms. Been there, done that, wasted the budget. Focus on the source, not the count.