You will need 8 buttons. Rowan used #A0502 from Groves & Banks

Finished measurements:
Back width: 18(20.5, 22, 24, 26)“ 46(51.5, 56, 61.5, 66) cm
Finished length: Approx. 21(21.5, 22.5, 23, 24)” 53(55, 57, 59, 61) cm
Sleeve length to undearm: 7.5(17.5, 18, 18, 18)“ 45(45, 46, 46,46) cm

Yarn used:
Rowan Alpaca Soft DK
Shade #202 Trench Coat 8(9, 10, 11, 12) balls