Abstract: A multiple-document detection system for an insertion machine comprises a plurality of document detectors, each detector being preset to a successive maximum allowable document-group-thickness measurement; a group of documents whose measured thickness exceeds the preset maximum allowable size for a particular detector will cause that detector's input line to change from a "normal" level to a "tripped" level. A processing means then determines the actual number of documents in a measured group of documents according to which of the successively preset detectors was the first to be "tripped". The processing means also receives a signal indicative of an expected number of documents for the particular measured group. The expected-number signal is sent, for example, by a reader, upstream of the plurality of detectors, which reads the expected number from a master document in the group.