To import multiple ICS files into Apple Calendar, save them in one folder on your Mac. Open Calendar, go to File > Import, then hold Command (⌘) to select multiple ICS files at once. Click Import and choose your destination calendar (select iCloud for syncing or On My Mac for local storage).

After importing, check for duplicates—especially when adding many files. For large batches, you can review files beforehand using the SysInfo ICS Converter Tool.
Read more: https://www.sysinfotools.com/blog/import-ics-to-apple-calendar/