I use LR5 on my Mac Desktop (Yosemite 10.10.1) and it works fine. I have downloaded LR5 to my new MacBookAir (Yosemite 10.10.1) and it will not open. Message says program in running on too many computers. I once had it on another MacBookPro (Yosemite 10.10.1), but no longer use that computer and LR5 does not show up in the applications.
What is the exact complete word-for-word error message?
Is this the Creative cloud version of Lightroom or the standalone (also called "perpetual license") version of Lightroom?
This must be the creative cloud version, otherwise you won't get that message at all. If so, simply log on to creative cloud on the new machine and Lightroom should work.