I wonder if there is any industry stats on what is an acceptable level of server errors e.g. crawl errors or total server errors or both, either as an absolute number or as a percentage of requests?
is both the percentage and the absolute number which should be your aim for the number of 500 errors that normal usage of your web applications and website will trigger.
More might be acceptable if the impact of such an unfulfilled request is limited, for instance when a client will simply try again at a later moment and continue to try until the request will have been completed successfully.
That will require analyses of which requests fail with an internal server error.