The maximum number of delivered Change Data Capture event notifications in the last 24 hours or daily, shared by all the clients is 50000. Clients include CometD, Pub/Sub API, and the empApi Lightning component only.

Reference Article:

https://developer.salesforce.com/docs/atlas.en-us.change_data_capture.meta/change_data_capture/cdc_allocations.htm

To check or monitor the usage, use the following queries:

SOQLs:

Delivered Events:

SELECT Name, StartDate, EndDate, Value 
FROM PlatformEventUsageMetric 
WHERE Name='CHANGE_EVENTS_DELIVERED' 
AND StartDate>=2022-08-31T00:00:00.000Z AND EndDate<=2022-09-01T00:00:00.000Z

The above query will return Change Data Capture events delivered on 31st August 2022.

Published Events:

SELECT Name, StartDate, EndDate, Value 
FROM PlatformEventUsageMetric 
WHERE Name='CHANGE_EVENTS_PUBLISHED' 
AND StartDate>=2022-09-01T00:00:00.000Z AND EndDate<=2022-09-02T00:00:00.000Z

The above query will return Change Data Capture events published on 1st September 2022.

Note:

Timings in the queries are GMT/UTC