How many syllables in Macbeth double double?

Answer 1

Double, double toil and trouble;

Fire burn, and cauldron bubble.

You're going to get some disagreement here. As written, diphthongs don't count, there are 15 syllables. Spoken rhythmically in performance, there could be 16 or 17.

Answer 2

Ten is the standard number of syllables for a line of blank verse, which is the kind of verse Shakespeare and many of his contemporaries habitually wrote in. There's a lot of it in Macbeth, e.g. "So fair and foul a day I have not seen." or "Creeps in this petty pace from day to day." But Shakespeare was not afraid to include lines with more or less than ten, like "Tomorrow and tomorrow and tomorrow." or "Convert to anger; blunt not the heart, enrage it." Curiously the witches tend to speak in lines of seven or eight syllables, unusual in a Shakespeare play (although the fairies in A Midsummer Night's Dream often use seven-syllable lines as well. It must be a magic thing.)