PEO's have their own minimum. Some will have a minimum 5 lives while others will go lower.
PEO's offer a multitude of services, not just group health. Their primary function is payroll but they also create a legal barrier between the (former) employee and employer to minimize employer liability. Many include EPLI in their package. PEO's handle the legal requirements (state & federal) of employee management. Many will offer WC and group life, medical, dental, disability.
There is a fair amount of abuse and mismanagement on the insurance risk side and many PEO's have lost access to WC and other insurance products. When working with a PEO make sure you understand what they offer and how it integrates with the employer.
How in the world could I work with a PEO? Wouldn't they just push me aside and take the client for themselves: lock, stock and barrel? What would they need me for?