The Schneider electric 174CEV30010 Modbus to Ethernet bridge is capable of addressing Modbus slaves from 1 to 247.
The actual number of devices which can be physically connected depends on the medium used and not on the fact it is Modbus. Some examples: RS232 is a 2 device point to point connection when using copper, however when using fiber (or other modems) it is possible to connect up to the maximum of 247 devices.
RS485 is multi-drop and supports 32 devices on a single network. When a repeater is used, on copper wire, the number of devices can usually be increased to 64.
Hope this helps.
Rohan Sood Applications Engineer National Instruments