The too-simple answer is "64" because the 1756-ENBT supports a maximum of 64 TCP connections, and each adapter will use one.
The real answer is "it depends"; it depends on how many analog modules are connected to those adapters, on how many HMI, Message, Produced/Consumed, or programming connections will be using the module, and on how fast the RPI values are set for the cyclic connections.
There are several important tools for configuring EtherNet/IP networks:
1. The big general user manual. Publication ENET-UM001 covers all the EtherNet/IP modules in the ControlLogix family. Get it from the literature.rockwellautomation.com website.
2. The EtherNet/IP Capacity Tool. It's just an Excel spreadsheet based tool for estimating the capacity of a ControlLogix interface for I/O purposes. Download it here:
http://www.rockwellautomation.com/solutions/integratedarchitecture/resources3.html
3. RSNetworx for EtherNet/IP. While you don't require this for configuring ControlLogix EtherNet/IP networks, if you happen to have it (like in the Professional bundle) it will give you very good offline estimates of bandwidth consumption.
If your 1756-ENBT won't handle all the connections you need, you have the option of multiple modules, or the more-powerful 1756-EN2T or 1756-EN3T modules.
