Iṭrād follows the chain from a named person to successive ancestors. Thus in the schematic expression “Zayd son of ʿAmr son of Khālid”, the sequence advances from Zayd to his father and then to the prior generation.
Three things matter: a coherent genealogical relation, orderly progression, and unforced wording. An interrupted or artificially packed series of names may fail to attain the rhetorical elegance of iṭrād.
The figure is about ease and sequence in naming, not mere quantity of lineage information.