An Emergency Action Plan is about one thing: getting people to safety when something goes wrong. It's the plan for the emergency itself — not for preventing it (that's a Fire Prevention Plan, which is a separate document — see our Fire Prevention Plan guide).
A written EAP is required for most employers above a certain size, and — this is the part people miss — it's also pulled in whenever another standard specifically calls for one. So you can end up needing a written EAP not because of a general headcount rule, but because a hazard-specific standard that applies to your work requires it. That's why "do I need an EAP" often can't be answered without looking at your whole hazard picture.
When required, an EAP generally has to cover:
Evacuation procedures and routes — how people get out, and to where.
How employees are alerted to an emergency.
Procedures for employees who stay to handle critical operations before evacuating, if any.
Accounting for all employees after evacuation.
Rescue and medical duties for those assigned them.