The molecular formula $C_3H_9N$ represents saturated aliphatic amines. It can form a total of four structural isomers, categorized by their amine degree:

Primary ($1^\circ$) amines:

1. Propan-1-amine (n-propylamine): $CH_3-CH_2-CH_2-NH_2$
2. Propan-2-amine (isopropylamine): $CH_3-CH(NH_2)-CH_3$

Secondary ($2^\circ$) amine: 3. N-Methylethanamine (ethylmethylamine): $CH_3-NH-CH_2-CH_3$

Tertiary ($3^\circ$) amine: 4. N,N-Dimethylmethanamine (trimethylamine): $(CH_3)_3N$

Therefore, there are exactly 4 structural isomers.