Oblivion only had 229 quests total. 11 were in an unfinished state.
Morrowind had 425 quests total. 7 were in an unfinished state.
Daggerfall had 248 quests total.
Arena only had 10 quests.
It seems to increase and decrease with each version. Lets hope the next release is an increase.
I was wondering, if one person can create 36 great quests over the lifetime of creating the product (about 3 years), which seems doable, about 1 great quest a month. 10 days to design, 10 days to implement, 10 days to test and rework. It could be using already made objects. Then it takes about 12 people only. So the length of the game is only limited by number of people they hire. So if they want 1000 quests, they just need to hire about 30 people. I only hope Elder Scrolls 6 will have at a minimum about 800 quests.