ISO 9001 questions

Document control requires that we have a method to identify changes in the document version. ISO 9001:2015 in clause 7.5.3.2 c) states “control of changes (e.g. version control)”. ISO 9001:2015 does not use neither “issue” nor “revision” in its text. Organizations that use issue no. and revision no. simultaneously, normally use revision no. to identify small changes in content, and use issue no. to either identify one big change in content or a kind of reset after five or ten revisions in a row. So, they are both used to identify changes but have a different scope of application.
No, it is not mandatory to have both, you can use issue no, or revision no, or even version no.
See answer to question 2
 

You can find more information about documentation below:

• New approach to document and record control in ISO 9001:2015 - https://advisera.com/9001academy/blog/2015/06/30/new-approach-to-document-and-record-control-in-iso-90012015/
• Some tips to make Document Control more useful for your QMS - https://advisera.com/9001academy/blog/2014/05/20/tips-make-document-control-useful-qms/
• Free online training ISO 9001:2015 Foundations Course - https://advisera.com/training/iso-9001-foundations-course/
• Book - Managing ISO Documentation: A Plain English Guide - https://advisera.com/books/managing-iso-documentation-plain-english-guide/
   

How many NCs (minor or major) are allowed in IATF 16949:2016?

Neither the IATF 16949:2016 standard nor the IATF Rule 5 have any requirements in this regard.

That is, no restriction has been defined for the number of major or minor nonconformities.