5
In a company of 5, this would be a disastrous turnover rate.
Seriously though, I'd say turnover rate should factor in what the turnover rate is after N years, and what rank of employee are leaving. If a company is losing 15% of its new hires, that's bad, but it would be really bad if they're losing even 10% of their mid-range staff, people who've been there for a few years, but for whatever reason won't stay with the company.