The number of open-chain amides (primary, secondary and tertiary, formamides included) of formula $\mathrm{C}{}_4\mathrm{H}{}_9\mathrm{NO}$, structural isomers only, is

Primary: butanamide, 2-methylpropanamide (2). N-Monosubstituted: N-methylpropanamide, N-ethylethanamide, N-propylmethanamide, N-isopropylmethanamide (4). N,N-Disubstituted: N,N-dimethylethanamide, N-ethyl-N-methylmethanamide (2). Total = 8.