If the external system is doing a simple REST API call into Salesforce (albeit many of them very quickly together, even in parallel), Salesforce will treat each call as a separate Transaction, and triggers will run on each item separately.
In general, multiple records are only processed together if a singular action was called on multiple records at once (...
In addition to @BrianMiller answer,
If one or more publishers publish Platform Event Foo__e as follows
T(0) - publish 10 Foo__e
T(1) - publish 3 Foo__e
T(2) - publish 6 Foo__e
then the trigger that subscribes to Foo__e could see anywhere from:
1 to all 19 (and anything in between) Foo__e in a single transaction as SFDC Platform Event subscription code ...