We allow you to create and link to as many sites and agents as you require. In order to manage server load and allocate resources as required, we offer you different packages which are based on "concurrent chats".

All this is, is the number of chats you're currently connected/chatting to. In other words, If you have 20 chats actively going on at once across all your sites and agents, you have 20 concurrent chats.

See also: What happens to chat boxes which fall outside the concurrent chats limit?