A Shakespearean sonnet has 14 lines.
The standard English or Shakespearean sonnet is usually organized into three quatrains followed by a final rhyming couplet. A quatrain contains four lines, so the structure is 4 + 4 + 4 + 2. The most familiar rhyme scheme is ABAB CDCD EFEF GG, although poets sometimes vary it.
The final couplet often delivers a conclusion, reversal, summary, or sharp change in perspective. That turn in thought is related to the poetic 'volta,' which can also appear earlier, especially around the beginning of the third quatrain. Shakespeare's sonnets generally use iambic pentameter, meaning each line commonly contains five iambic rhythmic feet.
A frequent confusion is between the Shakespearean sonnet and the Italian, or Petrarchan, sonnet. Both forms contain 14 lines, but the Shakespearean pattern uses three quatrains and a couplet, while the Italian form typically divides into an octave of eight lines and a sestet of six.