It's difficult to count the errors because some time a lot of minor things cause of errors.
We need to concentrate on Google latest updates and try to correct onpage strategies.

There are various types of errors in SEO.

Indexing Errors
Crawl Errors
W3C Errors
Canonical errors
Missing Meta title and description
URL structuring

It depends upon the Linkbuiding activities.
1) content penalty
2) poor links structure
3) spamy links

Instead of trying to find out how many errors one can possibly make in SEO, you should be interested in how to correctly optimise your site in the current SEO landscape.