Number of bytes receives not the same after each cycle

[Lesiok]

This is normal symptom for each data stream device (serial port, TCP socket etc.). 9 characters in packet is your private assumption. Data stream device do not know about it. You should read stream of data, analyze it and divide for logical packets.
By the way your protocol is very weak. How do you know that for example 01 is the number of the next packet and not its content ?