The simple answer is that your license will activate most Versions of RSLogix 5000. If you have v20 but need to open a project written for v17, you can install v17 off the installation DVDs.
While you need to install support for each Major Revision separately, in general you can have many Versions of RSLogix 5000 installed, but since the product has spanned the lifetimes of several major Windows versions, there are some limitations in OS support. I have versions 12-19 on an XP virtual machine, and 19,20,21,24 on a Windows 7 VM.
There are at least three more complex answers:
1. Very old versions of RSLogix only supported the old diskette-based activations. Anything older than Version 9 won't be activated by your FactoryTalk Activation. Unless you're trying to work on a system that's more than about 10 years old, this is not a problem.
2. You might have an Edition mismatch, not a Version mismatch. RA sells reduced-feature "Compact" and "Mini" editions of RSLogix 5000 licenses that support only CompactLogix and not the full size 1756 ControlLogix. You would need a license upgrade if that's the case. The Help ->About in your RSLogix 5000 installation should tell you which Edition you have.
3. The contents of the installation DVDs has varied over the years, as the bundled software has grown, RA started including fewer older versions on the DVD, so you need a TechConnect support agreement to download them.
The details of what was on each DVD are in
RA Knowledgebase article 29461 (Access Level: TechConnect).
If you have v20, the odds are good that your DVD includes all the versions back to v12 or even v10.