The maximum number of concurrent ACD interactions (whether calls, chats, e-mails, etc.) an agent can receive is determined by the ACD utilization settings for the queue. See Interaction Administrator help for more detail.
Keep in mind that the number of simultaneous chats an agent can handle will depend on what other work has to be done to facilitate each chat session (looking up information, performing tests, etc.). There comes a point where handling multiple interactions causes a decrease in efficiency just because it is hard to juggle and keep track of what is happening on each.