1) It depends on the current hiring needs and the position (Intern, analyst, associate). At McKinsey it may vary from 20 to 24 correct answers out of 26 questions. You'll never know the exact number though
2) Since the questions are different it's impossible to check the time after each question. What you can do is to calculate how much time you should ideally have left after every 4-6 questions and memorize this table. In that case, if you feel that you are running out of time you can prioritize questions and skip the hard ones to get back on track.
3) Here are some tips:
- Try to find as many people in your country as you can who have done the test. Ask them for the topic, industry, and questions they remember. For example in one of the countries there was a test about airlines and reading a couple of articles / company reports / wikipedia artticles with industry terms could significantly improve performance
- Try to practice tests with strict time-management. McKinsey PST from the official web site and all mck and bcg -like tests you can find online
- Buy Viktor Cheng test prep program - best materials I've seen so far for Mck PST
- Additional resources can be GMAT Integrated reasoning from the official guides and apps if you feel that you are not p[erforming well
- Practice your math. Check exercises on Cheng website. Key things - multiplication of 2 digit numbers, operations with zeros and division (Learn division table, ie. 1/8 = 12.5%, 8/9 = 88%, Learn up to 8/9).