This life, 4 projects (5 if you count the parent in the merge).
I'd recommend backing up at least weekly, assuming all your projects are worked on each week. Obviously if you only update one every few months, you'd only back it up when you update it.
If you are making lots of significant changes to a project, then daily would be good - it depends how much work you want to red-do if your PC explodes.
And I always do a backup immediately after releasing a production version of content.
I'm not actually using Flare, so I just zip using the Windows zip functionality. I'd recommend zipping, as it reduces file copy times and total file size. It's also a certain amount of protection so you don't accidentally modify your backup. (I think Flare allows some integration with the build process, so you might be able to write a script for, say, your production target, which automatically zips your source and copies to a network location using something like RoboCopy, or whatever tools you have access and support for. But that's hypothetical. Just throwing it out there as an idea.)
I'd say I access my most recent one or two backups a few times a year - sometimes I run a script and screw things up, or maybe I make a significant change that is then pulled from the product, and it's easier to get the old copy of a few topics from backup. I don't often go back to really old backups; one case might be to check when a particular change was made or if the content has always been that way, if an SME queries something.
This works reasonably in a small team. For larger teams I'd really recommend source control (does the organisation ban developers from source control? why is the documentation source any different from application source? for example.) You have to remember to back up this way, so a risk is you forget and you waste a heap of time when something is corrupted and you don't have a recent backup. Larger teams have more points of failure in this regard. Make sure you set up a standard naming convention and dedicated folders, so that everything is in a standard location with a standard name, so you can quickly and easily find the latest version. You might also like to have a slightly different name for your draft versions and your final production version, in case you need to go back to a previous production build. If there is a way to automate as much of this backup process as possible, I'd recommend it. For example, maybe you can get developers to write a script, so your team can just double-click on desktop icon and it automatically zips the project with the standard name and copies it to the network share.