How many of the ex-employers does reference/background check check on?

Background checks are usually done after the offer has been extended and accepted and they can be very comprehensive. They are not part of the evaluation whether or not to extend an offer.

Depending on country, checks often include police records, credit history, residential status for internationals, etc. Considering how thorough this process is, expect all your ex-employers to be contacted at this point.

Regarding the nature of the contact with your ex-employer, this will usually just be with HR where they confirm dates of employment and whether you did anything bad e.g. breach of contract, misconduct, etc. They will not discuss your performance with your former supervisor etc.

Please note that the above is very general and may of course vary between firms, countries, etc.

There is no blanket answer and it will vary with companies, countries and offices. For e.g. in UK, some Consulting companies do 5 year back ground, some 7 and some 10 years.

This is such a vague question that there will not be a satisfying answer for you. This depends on country/region, role, company, etc.

Just assume you will be background checked and tell the truth - it's as simple as that!

Bakground check: most of them will do it. Assume they will confirm you worked for your previous employers and the dates you worked there.

Reference check: no one will do this.