Well I personally loved the tapes, and watched them a few times for fun since my friends also did. it was never a conscious thing for me of having to "learn" the episodes, i could quote etc just coz i found them funny.
Nevertheless i recommend you do know all of them well enough to be able to discuss them. Not because u'd be asked to specifically refer to an episode but because each episode does have different ideas in it/reiterate and support ideas in the others. You can of course choose to focus on only 3 and just be able to manipulate the ideas in them sufficiently well.
There are 2 ways of structuring your Frontline responses (I found):
1 - introduction, para on first episode u're using, para on 2nd episode u're using (3rd etc...depends on ur timing), para on related material piece 1, para on related material piece 2, conclusion.
I found this way made sure you got down wat u wanted to say, and is good if you don't know the material really well.
However I realised i got much better marks when i used this way:
2: introduction (expounding 2-3themes that relate to your question, name the episodes u'll use/related material)
para on first theme (integrating all relevant parts from various episodes/related material)
para on 2nd/3rd themes (ditto)
Conclusion - bringing it all together.
If you studied and wrote your notes the way i suggested above, where you break the episodes into 'themes', its really easy to do the second essay construct. When i read over the essays from the first way that i did it, and compared it to the second method, the second way helped me make a much more cogent, convincing and fluid argument, and i really recommend it.
So no, there is no limit to how many episodes u can mention, but make sure that whatever u do use is relevant, supports ur purpose and u can speak well about it - don't use things for the sake of it and don't waste your time either. Quality is far superior to quantity!