Twenty errors? Ummmm… If I come across more than four or five, I send it back to the author with a gentle reminder to proof it further themselves. When you get into that level of corrections, on top of form editing recommendations, you're firmly into the paid editor's territory. You shouldn't be expected to do that for free.

I recommend you lay out detailed instructions about style and grammar requirements, and tell your contributors you will bounce their work right back to them for exessive errors.

I don't edit for content. I have no issue with correcting a very few spelling or punctuation errors. I might make a suggestion here or there for form and structure. However, I expect my contributors to submit professional documents that need little more than very minor edits and/or a bit of polish.

If they want more than that, you deserve to be paid. Most of the editors I know charge an average of 30¢ per word for copy editing/proofreading. Full developmental editing goes for $10K or more per book.