The "best" bitrate on DVDs will of course depend on quality, source, type of material, etc. For instance, VHS sources usually require a higher bitrate than digital TV rips to compensate for noise and similar.

But assuming these are digital transfers of high quality (not too much noise or grain), I would personally recommend putting 5 episodes per disc. That way, it should be under two hours per disc, which is a common standard on retail DVDs. If you use a good encoding software (such as HcEnc), that should then look very good.

The exact bitrate will of course depend on the exact running time, number of audio tracks and the audio bitrate (are these episodes in mono or stereo?). But the video bitrate should then be somewhere between 4100 and 4750 kbit/sec, which are pretty normal bitrates compared to many Hollywood retail DVDs.