The total number of gates you can close is 60 (50 random gates, 9 standard gates, and the Great Gate). There are a total of 100 fixed locations where gates may open.
But there are still two Oblivion realms you can enter after the main quest has finished. Both Boethia's Daedric quest and Peryite's Daedric quest send you to Daedric Realms after the conclusion of the main quest, if you choose to do those quests later.
::EDIT:: Sigil Stones (and everything else that is random, such as loot, enemy levels, etc) is determined at the time you activate the Gate. If you save before activating a gate you can "shop" for different sigil stones and other loot.
It seems you've contradicted the other two informational replies in this thread.
1) There are not up to 100 gates available to explore.
2) The sigil stone is determined at the time you enter the Gate instead of at the time you grab the stone.
b
) Is just the level of the sigil stone determined for the first time entering the gate (ascendent, transcendent, etc.) or is the type of sigil stone itself determined as well?