Negatively charged clusters of N-monosubstituted amide molecules [(XCO-NHY)_n^-; X, Y=H, CH₃, C₂H₅] were formed by injecting slow electrons into the collision region of supersonic expansions. In addition to the dipole-bound dimer species reported by C. Desfrancois, V. Périquet, S. Carles, J. P. Schermann and L. Adamowicz [Chem. Phys., 1998, 239, 475], larger clusters up to n=50 were created for the first time. Efficient formation of trimer anions was observed for ethyl substituted amides. With a threshold size of n=6 or 7, prominent peaks of magic numbers were found for clusters of N-methylformamide and N-methylacetamide, whereas smoothly undulated size-distributions are developed for the ethyl substitutions. A pattern was found in the periodicity of the undulations, of which the span extends as the substituents become bulkier. A hypothetical structure for large clusters in a spiral arrangement is proposed from aspects of electrostatic interactions to make an agreement with the pattern.