You will need 4(4, 5, 5, 6) buttons

This pattern also has instructions for a collared cardigan and a crew-neck pullover.

Finished measurements:
Chest circumference: 16(17, 19, 21, 23.25)“ 38(42, 48, 53, 59) cm
Finished length: 7(8, 9.5, 11, 12.5)” 18(20, 24, 28, 32) cm
Sleeve seam: 3(4.75, 6, 6.5, 8)“ 8(12, 15, 17, 20) cm

Yarn used:
King Cole Cottonsoft DK
Shade 713 Lilac - 1(1, 2, 2, 2) balls