A backlog is an exam that you could not clear in the first attempt. If you have registered for the exam and could not attend due to sickness or any other issues, it is counted as a backlog. However, in the case of absenteeism, you should always confirm from your academic institution whether they mark an absent or write zero in the score column. It is essential to appear for the exam the next time under any circumstances in order to avail your degree from the institution of study.
The number of times you have appeared to clear a subject has got everything to do with the number of your backlogs. For example, if you had to appear 5 times to clear a backlog in one subject, you will have 5 backlogs (including number of attempts). If you had to clear 5 subjects and you did that in 1 attempt each, you still had 5 backlogs.
A backlog certificate is a sheet issued by your academic institution that has details about your backlogs. In case you had no backlogs, your institute would not provide you with any such certificate. However, if you have backlogs, you would be asked to submit a backlog certificate to certify your academic records along with academic transcripts.
There is no specific answer to this question. Backlogs are accepted in almost every country. Nevertheless, there is always a limited number of backlogs that are counted in every region. Every college, national or international, wants to select the best candidates for sure. What students need to understand in this regard is what makes their profile strong. There are many good universities across the globe that select average profile if they find them suitable for the kind of courses they have to offer.