What meter is used in Sonnet 18 of "Twelfth Night"?
I think you've got a little muddled here. Shakespeare wrote "Sonnets" and also "Twelfth Night": though "Twelfth Night" is a play, not poetry.
Shakespeare's "Sonnet 18" is below - and it's written in iambic pentameter. (de DUM de DUM de DUM de DUM de DUM - five weak beats alternating with five strong beats). It ends in a rhyming couplet.
Shall I compare thee to a summer's day?So long as men can breathe or eyes can see,
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date;
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance or nature's changing course untrimm'd;
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st:
So long lives this, and this gives life to thee.