I have all ready installed Creative Cloud on my macbook. Now I have also a iMac and I want to use for example lightroom also on the iMac. Can I use my license on both computers?
Yes, your subscription allows for having two working installations.
Does that apply to all software applications in the subscription? I.e. are they treated as a whole unit?
Or can we have Photoshop on computers 1 & 2 and Acrobat Pro on computers 1 & 3?
A whole unit. The desktop application is needed to run the cloud installation and that can only be signed in on two machines at any time.
You can install on as many machines as you like. Of all those machines you can only have two signed in for use. Of those two machines you can only be using the subscription on one at a time.
Do you have to log out of one of them to use the other? I have a license for my mac mini and another license for my mac laptop - rather than pay for two licenses I could eliminate one?
you can install the cc desktop app from here https://creative.adobe.com/products/creative-cloud on as many computers as you own and use it to install all (or some, if you want) of your cc subscription programs. the only (insignificant for internet connected computers) limitation is that you can sign-in (and therefore start your adobe programs) on, at most, two computers at any one time.
because you can sign out from either (or both) signed-in computers using a 3rd computer, this could not (imo) be easier.