I am asking this question again as I might have the answer! I found somewhere previous posts from DTaylor and Tim both saying that as long as it's for your own personal/single use that you can install TC MacPro on as many machines as you have. So as I now have a new 16"MBP as well as my old iMac I would install V12 on both. I have a slowness issue (which I will raise in a new post) on V11 so it was a good excuse to get V12. It successfully installed on both machines. I then tried installing V12 on my old MacAir as the speed issue I am having is still there. I thought it might be the OS. iMac and MBP both run Catalina and the Air is running Mojave. V12 installation on the Air failed saying I had reached my limit ie 2 x machines. So I don't need a reply (unless I have got this statement wrong). So the answer is that you can install TC on 2 x machines but not a 3rd !