The Elder Scrolls IV: Oblivion features a maximum of 60 Oblivion Gates that can open during any given playthrough. These gates open as portals to the Oblivion realm, allowing demons to cross into Cyrodiil. Among these 60 gates:

• 20 gates have fixed spawn points.
• 40 gates spawn in random locations from a pool of over 100 possible locations.

You don't have to close all 60 gates to complete the main quest; completing approximately 20 gates is enough to finish the main story and stop new gates from spawning. The first gate, located at Kvatch, serves as a tutorial, and subsequent gates appear as you progress through the quest line. Once all or a significant number of gates are closed, no more will open in the world. Closing an Oblivion Gate involves entering it, climbing to the Sigil Tower inside, and destroying the Sigil Stone at the top which powers the gate.