The Question Behind the Question
The real decision isn't "how many pages do I want" — it's "what does a visitor need to do on my site, and how many distinct things am I actually offering?" That answer usually points to the right structure on its own.
When a One-Page Site Works Well
If you offer a single service or product, a one-page site that flows from introduction to details to a clear call-to-action can convert just as well as a larger site — often better, since there's no risk of visitors getting lost between pages before they take action.
When You Need a Full Multi-Page Site
If you have multiple distinct services, need dedicated pages that can rank individually in search results, or want a blog, portfolio, or team page, a single page becomes cramped fast. Multi-page sites also give search engines more individual pages to rank for different keywords.
SEO Is Often the Deciding Factor
A one-page site can only really target one primary set of keywords. If you want to rank for several different services separately, each with its own dedicated content, a multi-page structure is close to essential.
You Can Start Small and Grow
It's completely reasonable to launch with a one-page site to get online fast and affordably, then expand into a full multi-page site as your offering grows — the two aren't mutually exclusive, just different stages.