We already track the number of statements per Wikidata item over time (Grafana), by looking at the wb-claims page property. Analogously, we can look at wb-identifiers to track just the number of external identifier statements, which is also useful information that’s otherwise hard to get (in WDQS or on the Toolforge replicas, the query times out, whereas on the stats machines it finishes within half a minute).
Relevant code: analytics/wmde/scripts.git, src/wikidata/datamodel/statements_per_entity.php and src/wikidata/datamodel/sql/select_statements_per_entity.sql.