No, there is not a minimum number of agents for the Legacy Enterprise plan in LiveAgent. The plan is billed per agent, so you pay only for the agents you have.
Benefits for larger accounts
Accounts with 10+ agents get extra benefits on the Legacy Enterprise plan, such as a dedicated account manager. See What are the benefits of the Legacy Enterprise plan? for details.
Availability of the Legacy Enterprise plan
The Legacy Enterprise plan is not available anymore, so it is not possible to change to it from another plan. If you are already on the Legacy Enterprise plan, you keep it and can still change your number of agents at any time.
Looking for the current Enterprise plan?
The current Enterprise plan is a flat fee that includes 80 agents. See Is there a minimum number of agents for the Enterprise plan? You can compare all available plans on our pricing page.