If the files/folders won't fit in one disc, it will show on the bottom that more disc(s) are needed. Or you can select less and manually put them onto multiple discs. If you need more than 2 discs (like the crazy 12 discs span shown below), STOP, you are way better off get an external hard drive.
Note that you cannot burn files >4GB with the defauly UDF/ISO 9660 format.
choose "Close disc" and click "Set as Default" the first time, it will remember next time.
Every once a while, also check "Verify after burning" to test the burnt disc. It may take a long time to verify though for some drive/disc/files,
click the big burn button (which does not look like a button at all)
If your backup is > 4.35GB, you will get additional prompts
Take about 11 min. to burn at 8x (without verification)