The number of gates, and their likelihood of opening, depends on how far you are in the Main Quest. You can install the mod at any point in the Main Quest, or before or after, and it should work fine.
You can get the mod here:
http://www.tesnexus.com/downloads/file.php?id=33533
Details on how it works:
Spoiler
In Vanilla Oblivion, 50 Random Gates and 10 Fixed Gates are the limit:
This mod makes it so all 90 Random Gates can open:
In Vanilla Oblivion, 50 Random Gates and 10 Fixed Gates are the limit:
- There are 10 Fixed Gates which always open: Kvatch, Bruma, Anvil, Bravil, Cheydinhal, Chorrol, Leyawiin, Skingrad, Fort Sutch, and the Great Gate.
- After the quest Find the Heir: Up to 25 random Gates can exist; each has a 25% chance of opening.
- After the quest Dagon Shrine: Up to 50 random Gates can exist; each has a 50% chance of opening.
- After the quest Paradise: Up to 20 random Gates can exist; each has a 25% chance of opening. (If more than 20 have already been closed then no more Gates open.)
- After the conclusion of the Main Quest: All Gates close.
This mod makes it so all 90 Random Gates can open:
- The 10 Fixed Gates still open on schedule.
- After the quest Find the Heir: same as in the original Oblivion.
- After the quest The Path of Dawn: Up to 50 random Gates can exist; each has a 40% chance of opening.
- After the quest Dagon Shrine: Up to 70 random Gates can exist; each has a 60% chance of opening.
- After the quest Bruma Gate: Up to 90 random Gates can exist; each has an 80% chance of opening.
- After the quest Great Gate: Up to 50 random Gates can exist; each has a 40% chance of opening. (If more than 40 have already been closed then no more Gates open.)
- After the quest Paradise: same as in the original Oblivion.
- After the conclusion of the Main Quest: All Gates close.