David has Airport Extreme routers at home, and he wants to know how much data is being used on his network. Leo says this is one of the drawbacks of the Airport Extreme routers: they lack some of the more modern features that other routers have. One way he could do this is by replacing the Airport Extremes with a more modern router that can monitor bandwidth. Leo uses the Asus AC3200 which has a built-in bandwidth monitor and can even tell him which computer is using the most.
There are apps like Bandwidth Plus that will track each computer's individual usage. He wouldn't be able to see the bandwidth of other computers that might be accessing his network, though.
He could also put something between his computers and the routers that could monitor bandwidth, like the Ubiquiti Edge Router. He'd have to put the Airport Extremes in Bridge mode, and use the Ubiquiti router in addition to it.