Sample dilution is dependent on the expected analyte levels in the samples.  For human serum, plasma, and urine samples, MSD recommends a minimum 4-fold dilution.  Tissue culture supernatants may require additional dilution based on stimulation and the analyte concentrations in the sample.  In general, you can use a higher dilution to conserve sample volume.  MSD also recommends testing at least two replicates per sample.