There was a similar question here How many paths can be purged with the API at once? , but the answer there points to a URL which does not exists any more.
No limits for URL purge is mentioned here
https://developers.cloudflare.com/api/operations/zone-purge , and Purge by single-file (by URL) here
https://developers.cloudflare.com/cache/how-to/purge-cache/#purge-by-single-file-by-url only have this:
The single-file purge rate limit for the Free subscription is 1000 urls/min. The rate limit is subject to change.