I received LR 5 for Christmas but have not downloaded it yet on my computer. I have had LR 3 for about 4 years. I am due to buy a new computer soon but need to know how many licenses/downloads I will get before downloading LR 5 on my current computer. I may also purchase a laptop in the next year so that would require 3 licenses altogether (current iMac, future iMac, future macbook).
You have one license.
You can download and install it on an infinite number of computers, as long as it is only installed on two of your computers at any one time, and in use on only one computer at any given time.
So in your case, current iMac, that's would mean its installed on one computer, no problem. Future iMac and future MacBook ... so if you uninstall from current iMac and install on Future iMac and future MacBook, it's now installed on two computers at a time, no problem, as long as you're not using Lightroom on both simultaneously. And if after that, you got two new computers and uninstalled Lightroom from the previous two, you're still good, it's only installed on two computers at any one time.