I am a novice in MODBUS protocol, and I am trying to understand what is the command memory and command memory count please?
How can we extract it from a wireshark capture please?

 

Check following manuals.
Modbus TCP is a Modbus RTU telegram (without CRC) in an Ethernet telegram. PI_MBUS_300.pdf gives a good description of all Modbus commands and command fields.

http://www.modbus.org/docs/PI_MBUS_300.pdf
http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf