A rulebook needs a lexicon

For example, 1.3.1 भूवादयो धातवः does not list a single root. It says: "bhū and the rest are called dhātu", and leaves the list to the Dhātupāṭha.

Sources: Abhyankar, A Dictionary of Sanskrit Grammar (1986), s.v. gaṇapāṭha, śākaṭāyana; Scharf, Sanskrit Library notes on the Mādhavīya-dhātuvṛtti (2009); Wikipedia, "Unadi-Sutras". Counts are from vidyut's data files.