If you have 100 or more eligible participants at the beginning of the plan year, then you likely need to have your plan audited by a qualified public accountant. For the purpose of the audit requirement, the participants are not simply defined as anyone who has an account balance. Participants also include persons who have met the age and service requirements for participation. However, there is some leeway offered to small plans via the 80/120 rule.
A plan that covers from 80 to 120 participants at the beginning of the year is in the gray area of the audit requirement. Basically, such a plan may choose not to have an audit if it did not need an audit in the previous year. If the plan has more than 120 participants, then it must be audited in the current year and subsequent years as long as it has above 100 eligible participants.