You'll 1st want to convert the files to MPEG2, balancing image quality against file size. You'll not like the results of trying to put more than 2 hours of video on a single-layer DVD. Once you know what the total file sizes are, then you can approach how best to author your DVDs.
AVI is basically raw video, DIVX, and an assortment of others, are MPEG4. For DVD-video you need MPEG2. The software will do the conversion, but that doesn't mean it can put an unlimited amount onto one DVD. I'll guess that your files are currently DIVX, or some other form of MPEG4, which has much smaller files sizes than MPEG2.
Some DVD players can play MPEG4/DIVX files, most cannot.
How long in minutes are each of the files?