There are two parts of the Answer:
1.) Cyber Security in critical infrastructure:
For safety reasons (not only in space but in all professions) you need physically separated networks when operating critical infrastructure. Think of the engineers in the 90s developing the station, they didn't want the crew to install WinAmp for entertainment on the same computer that controlled the thrusters of the Russian module (now pointing on an other module). So playing this infected MP3 downloaded with emule will not cause (worst case scenario) the loss of 13 astronauts, the ISS and a Shuttle. So only the American part has 3 network layers and because you do not trust anybody (not because of politics but because you pay attention in cyber security classes) the American network is separated from the Russian and European and Japanese. They have multiple networks on their own. Additional there is a network for the "smartphones" of the crew so when they Skype with their families the funny meme they get cannot infect an important network. ---> Plenty of networks, so you need plenty of Notebooks to access them
Because the time of an astronaut is costly, nobody wants an astronaut to wait till another one have finished a task and can pass on the laptop. Laptops on the other hand are cheap. So it is maybe not important if there are 70, 80 or 90 laptops, but when a crew-member needs one, there have always one to be free and available. ---> Plenty of more notebooks
As I said, it is a phenomena, not only in the ISS. In a bank, the computer where an employee is writing a letter to you is not (directly) connected to the network where the account data is running. In power plants offices have other networks than the control stations etc...