Is there a reason Shakespeare always uses a 5 Act structure in his plays? The Roman poet Horace says in his Art of Poetry that 5 Acts should always be used, and the Roman playwright Seneca, who was the model the Elizabethan playwrights used for tragedy, held to a 5 Act Structure. But other than following a convention, are there any reasons, practical or otherwise, for why Shakespeare's plays are all made up of 5 Acts?
Shakespeares plays contained within them between 9 scenes to 42 scenes, so having anywhere from 20-40 scenes divided into 3 Acts would leave each Act with too many scenes. Henry IV Part 1 and 2 each have 19 scenes, Henry V has 23 scenes. Unlike the Greek and Roman plays which took place in one location during the course of a single day, Shakespeare's plays took place in many locations sometimes vast distances apart, like in Henry V Macbeth, sometimes different locations in a single city, like in Romeo and Juliet. Hamlet has 20 scenes, Macbeth has 28 scenes, King Lear has 26 scenes, Richard III has 25 scenes, and Romeo and Juliet has 24 scenes. The most amount of scenes an Act will have is around 7. Some has2, 3, or 4 scenes, even 5 scenes, and sometimes even only 1 scene.
Comedies like The Two Gentlemen of Verona, with 20 scenes, the Taming of the Shrew with 12 scenes, also have 5 acts. Even if The Taming of the Shrew only has 2 scenes in Acts 1, 3, 5, 1 scene in Act 2, and 5 scenes in Act 4.
It seems that for his comedies modelled after Italian examples, 4 Acts would have been too few, remembering that you would have to fit the same amount of scenes into a less number of Acts, which would only seem to complicate things both from a practical perspective and an aesthetic one. It seems quite alright that some Acts only have a few scenes. A short play between 9 and 18 scenes will have 2-3 scenes per Act. Average length plays with 23-27 scenes will have 3-7 scenes in an Act. A play like Romeo and Juliet has 24 scenes. Act 1 has 5 scenes, Act 2 has 6 scenes, Act 3 has 5 scenes, Act 4 has 5 scenes, and Act 5 has 3 scenes. A comedy like Tweflth Night has 18 scenes, 5 in Act 1, 5 in Act 2, 4 in Act 3, 3 in Act 4, and 1 in Act 5.
Numbers do not necessarily mean length. Hamlet has only 20 scenes. Macbeth has 28 scenes. Titus Andronicus has only 14 scenes, but much happens within that play. Comedy vary between low number of 9 in Love's Labour's Lost or A Midsummer Night's Dream, 11 in A Comedy of Errors, 12 in The Taming of The Shrew, to 17 scenes in Much Ado About Nothing, 23 scenes in All's Well that Ends Well and The Merry Wives of Windsor.
If Shakespeare were to divide a play of 25 scenes into 3 Acts, each Act would have far too many, and a 4 Act structure would not have the structure of having a clear Middle Act. Would Act 2 or Act 4 be the Middle Act? With a 5 Act structure, the 3rd Act is always very neatly the middle part of the story. It gives the play and the story a central act with many scenes for the various characters involved.
Not all playwrights would stick to this model, as Anton Checkhov was fond of using a Four Act structure, and modern plays are experimental in their narrative structures. 1 Act plays are also a beloved genre of play. If Shakespeare were alive today he might very well experiment with many different forms, and he might also write 1 Act only to see that he had to much more of a story to tell and would go on to write 3 or 4 more.