If you are just going to put the avi files onto the dvds as data, then you can fit about 12 350mb avi files (real target size on a blank dvd is ~4.3 gb).
Many dvd players can playback avi files that are burned to a disk in this manner. You need to make sure that your player has this capability. The most compatible codecs used would be xvid or divx in an avi container file.
If you are going to convert the avi files to actual dvd-video format, you won’t be able to fit nearly as many on the dvd. This is because the mpeg2 codec used in dvd video is simply not as efficient at compression as divx or xvid. I normally try to get the best quality possible if I’m going to all the trouble to convert, so I don’t put more than 3 on a disk.
You can also use a bitrate calculator to estimate how many to put on the dvd. Here is an online calculator at videohelp.com: http://www.videohelp.com/calc.htm
Enter the running time of the combined video. Set the type to DVD. Vary the time, and watch the bitrate change. With mpeg2, I don’t like to go below 4000kbps, but that isn’t a hard and fast rule. You can get by with two and a half hours of video on the disk.