We present an experimental study on the high-harmonic yields from the ions in the laser-ablated plumes of various metal targets (W, Mo, Cr, Cu, Ni, Fe, Ag and Mg) with the purpose of comparing their ion density and single-atom response. The harmonic yields as a function of medium length are measured and the results are fitted against a theoretical model to extract the coherence length, absorption length and strength single-atom response (in arbitrary units) of different harmonic orders for each target. It is found that the coherence lengths decrease monotonically as a function of harmonic order for all targets. Ion density of the generation media are estimated by the trend of the coherence length as a function of harmonic order. Qualitatively, targets with lower melting temperatures seem to produce laser-ablated plumes of higher ion density, vice versa. Also, the strength of the single-atom response of the metal ion species with only one electron in the outermost subshell are weak compared with the other targets considered in this study.