Summary: Haven't actually counted lately so I took a walk around the house and looked. I currently have three M570s in use plus another on an old machine that I'm not using at the moment. And I just installed one on my ex-wife's new laptop. They are all on laptops except for my desktop machine where I use it paired with a Logitech K320 wireless keyboard using the Unifying software. I've been using Logitech Trackballs since DOS days and the M570 is the best of the bunch.