https://support.apple.com/HT202158 says:
Calendars and Reminders
- Total number of calendars, events and reminders: 50,000*
- Maximum combined number of calendars and reminder lists you can have: 100*
- Maximum size of all calendar and reminder data (including attachments): 1 GB*
- Maximum number of characters in a reminder title: 1,000*
- Maximum size of a calendar event (including attachments): 20 MB
- Maximum number of attachments per event: 20
- Maximum number of attendees you can invite to an event: 300
- Maximum number of people with whom you can share a private calendar: 100
* If you have upgraded your iCloud reminders on a device running iOS 13, iPadOS, or macOS Catalina or later, these limits on reminders do not apply. Find out more about the Reminders upgrade process.
As to the number of events I found this in a MacRumors forum:
Open Calendar then go to File menu then Export then Export... and export the calendar as a ICS file to your desktop. Now use Textedit to open that file. Notice how each event is in the format SUMMARY:Bob haircut for example. I just did a command-f and entered summary as the search term and to the right it tells me how many time the word summary appears in the file. That should be the number of events you have.
You would have to do this for each separate calendar. It's not something I can try out but someone else in that forum seemed happy with it.
If you are inside these limits you should contact BusyCal for clarification.
