Short answer: it depends on what you invoice under, not on whether you have employees.
The existing rule of thumb is "if invoices, staff or credentials are involved, it's an Organization" — but that leaves freelancers and sole proprietors in a genuine grey zone, since many invoice clients without ever hiring staff.
Use an Organization UIID if: you invoice under a registered business name or a separate tax ID, you plan to apply for Verifier status later, or you might eventually add contractors or staff under the same entity.
A personal Individual UIID is usually enough if: you invoice simply under your own name and personal tax ID, and you don't expect the business to grow into a separate legal entity.
Why this decision is worth getting right early: registering a business under a personal identity and migrating later is described elsewhere in this help center as a common and avoidable mistake — the same applies here, just at a smaller scale.
Still unsure? Write to [email protected] describing how you invoice today; this is exactly the kind of edge case worth a quick check before you commit.