This article describes how many meters that can be connected to the Gateway.
Anybus M-bus Gateways
How many meters can I connect to the gateway?
The gateway support up to 20/80 loads, not meters. One meter usually consumes between 1-4 loads so the amount of meters that you can connect is depending on the number of loads each meter consumes giving you the total number of loads on the bus. One load is 1.5mA.