That’s labor insurance, not employment insurance.
If it’s a buxiban then it’s neither a company nor a business (the word firm or 行號 should have a broader meaning than business or 商業, but according to the MOL it doesn’t), ergo the 5 employee rule doesn’t apply, so the only thing to make it compulsory is the existing registration. So with existing registration, even if the number of employees currently registered is zero, even if it’s always been zero, it’s still compulsory, but if the registration never occurred in the first place, it’s not.
On the other hand, if the business is a de facto buxiban but registered as a company (or as a business within the meaning of the Business Registration Act) instead, then it’s shot itself in the knee, because the five employee rule applies after all.