Exempt organizations annual reporting requirements - Form 990, Parts I-V: Reporting number of volunteers

How should the number of volunteers be reported in Part I of Form 990?

Make a reasonable estimate of the number of persons that did any type and amount of volunteer work for your organization during the tax year, not including your employees who may have done volunteer work in their spare time. An organization may, but is not required to, explain on Schedule O PDF how it determined the number of volunteers reported in Part I.