Number of quests in oblivion

Due to the 'Radiant' quest system, there is technically an infinite amount of quests. Basically the game has a database of quest givers, tasks and locations, and can randomly construct a quest for you out of these. So for example, speaking to a bartender will get you a 'Jarl's Bounty Notice' quest where the local Jarl will want you to kill a bandit leader, or dragon, etc. The game will occasionally repopulate cleared areas for these quests or send you on an identical mission to one you've done before.

However as well as all those random ones, there are over a hundred scripted quests for different factions and the main storyline. A complete list of every scripted quest and radiant quest variant can be found in the related link.