It's pretty clear that Midna destroys the Mirror of Twilight to separate the Twilight Realm from Hyrule, since she says, "... Light and shadow can't mix, as we all know. But... Never forget that there's another world bound to this one." Referring to the events that gave progressed thus far in the game.
The dialogue of before she breaks the mirror goes: "Thank you... Well, the princess [referring to Zelda] spoke truly — as long as that mirror's around, we could meet again..." After this she tearfully says "Link... I... See you later..." and then shatters the Mirror of Twilight with one of her tears.
Since she explicitly says that mirror and not a mirror it would imply that this is the only mirror connecting the two worlds.