Like how many sets of registers can be configured in Modbus TCP/IP to MQTT protocol, how many registers are in a single call, and the time for each call.
You'll have to be a little more specific. How are you getting the MODBUS registers? How are you transferring them to MQTT? What kind of channel are they going over? What is your MODBUS device capable of? There's quite a bit of info that can change the answers you're looking for without knowing what to provide.