This is what I found out.
Each MB_Client to MB_SERVER connection requires a seperate CONNECT_ID. There are a total of 8 connections that can be active at one time, as Pegaia stated.
If you are using multiple MB_CLIENT instructions to connect to the same MB_SERVER, lets say one to read holding registers and another to write holding registers to the same PLC, they can then have the same CONNECT_ID, but will require different IP_PORT numbers. The PLC in this case will require two MB_SERVER instructions, each with their own IP_PORT number. You also cannot have more than one MB_CLIENT instruction, with the same CONNECT_ID, sending a request at the same time. You will need to handle the sequencing in your program so that only one of these are sending a request at a time.
If you need to communicate with more that 8 Modbus Servers, as I did, this is possible if you follow these guidelines.
1. Only 8 Client/Server connections can be active at a time.
2. You must "Disconnect" one or more of the MB_CLIENT instructions to free one or more of the 8 available connections by setting the DISCONNECT input on the MB_CLIENT instruction to TRUE.
3. Make sure you do not disconnect the client while it is processing a request and is in the busy state. This is accomplished by monitoring the static bit "CONNECTED" that is located in the clients DB and the output bit "BUSY".
4. Once the 8th client TCP connection is disconnected, you will be able to "connect" your 9th MB_CLIENT connection.
Here is an example of how to write the logic to Disconnect and ReConnect the MB_CLIENT