M258/LMC058 can implement both Modbus TCP client and server; also can work as a Modbus TCP Slave device.
1) If the controller works as a Modbus TCP client, you need to add function blocks to read/write a server:
READ_VAR: Quantity is the number of objects to be read:
"    1-125: registers (MW and IW types)
"    1-2000: bits (I and Q types)
WRITE_VAR: Quantity is the number of objects to write:
"    1-123: registers (MW type)
"    1-1968: bits (Q type)
WRITE_READ_VAR: WriteQuantity is the number of objects to write:
"    1-121: registers (MW type)
ReadQuantity represents the number of objects to be read:
"    1-125: registers (MW type)

2) If the controller works as a Modbus TCP server, there is no restriction for the number of being read/write, it depends on the restriction of the Modbus TCP client.

3) If you configure Ethernet to Modbus TCP Slave device (It serves as a gateway for an external I/O scanner configured as the Modbus Master, and allows this scanner to exchange data with the controller without interfering with the operation of the Modbus server on the controller.), size of Holding Registers (%IW) and Input Registers (%QW) is 2...40 bytes.

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