I'm not sure it the OP is asking about speaking parts, or a musical actually designed for a small cast. for instance, I've seen a production of Little Women with many cast members. Dancers, townspeople, etc.
"Through The Sacrifice You Made, We Can't Believe The Price You Paid..For Love!"
Well [title of show] is technically 5, seeing as the keyboardist has a small role (or at least he did in the Landor production) Tell Me on a Sunday is a one woman show. Rocky Horror is 13 people.
2016 These Paper Bullets (1/02) Our Mother's Brief Affair (1/06), Dragon Boat Racing (1/08), Howard - reading (1/28), Shear Madness (2/10), Fun Home (2/17), Women Without Men (2/18), Trip Of Love (2/21), The First Gentleman -reading (2/22), Southern Comfort (2/23), The Robber Bridegroom (2/24), She Loves Me (3/11), Shuffle Along (4/12), Shear Madness (4/14), Dear Evan Hansen (4/16), American Psycho (4/23), Tuck Everlasting (5/10), Indian Summer (5/15), Peer Gynt (5/18), Broadway's Rising Stars (7/11), Trip of Love (7/27), CATS (7/31), The Layover (8/17), An Act Of God (8/31), The Curious Incident of the Dog in the Night-Time (8/24), Heisenberg (10/12), Fiddler On The Roof (11/02), Othello (11/23), Dear Evan Hansen (11/26), Les Liaisons Dangereuses (12/21) 2017 In Transit (2/01), Groundhog Day (4/04), Ring Twice For Miranda (4/07), Church And State (4/10), The Lucky One (4/19), Ernest Shackleton Loves Me (5/16), Building The Wall (5/19), Indecent (6/01), Six Degrees of Separation (6/09), Marvin's Room (6/28), A Doll's House Pt 2 (7/25) Curvy Widow (8/01)