Contacting A Professor for Master of Science (Thesis) I want to know whether or not the professors are generally interested to supervise students in their M.Sc. (Thesis) program.

How many students they may supervise?

Do professors insist that we know everything about their research work?
Do they provide us some time in case we don't satisfy the knowledge of their research work completely or do they deny to supervise us right away?

If a professor is ready to supervise, then does that guarantee the admission (assuming we meet all admission requirements)?
Professors get credit with their departments for supervising graduate students and getting them through to their degrees. Depending on the department, university, type of program (is the master's degree terminal? etc.), and country, the importance of supervising a master's student for a professor relative to a PhD student can vary a lot. In some places the PhD students may be all that really counts towards the professor's direct incentives, or they may be more or less equivalent. In a terminal master's program, the master's students are everything (if that isn't obvious). I don't know of anywhere where a master's student would count more than a PhD student towards a professor's supervision requirements, but it might be common in some professional programs that also grant PhDs.

Now, that being said, there are other incentives for professors that master's students can affect. Professors generally need to publish peer-reviewed works, and master's students can definitely contribute to that. Perhaps less so than PhD students, but most master's students are working towards at least one significant publication. Professors also must staff the grant-funded projects that they win and produce whatever deliverables they promised (or at least try to), and master's students are hired for this purpose as well. 

Typically, professors may have as many students as they can support with funding. That may be grant funding or departmental funding, but I know of no hard caps. Obviously, departments will want students to get good supervision, so if a professor has an inordinate number of students, say 100, then a department chair might try to slow them down or otherwise act to insure that the students are adequately supervised. On the other hand, departments tend to stay out of the way of extremely well-funded professors.

Professors may decide how to select their students in almost any way they choose as long as they do not discriminate based on legally protected statuses in their country (e.g. race, sex, religion, etc.) and as long as they comply with their university's local regulations. My PhD supervisor took me on knowing, since I was applying to the same university as my undergrad, that I knew virtually nothing of his research. I didn't have the mathematics, fluid mechanics, or really even the programming skills to contribute strongly on day one. That being said, other professors may do things differently, especially for master's students who are expected to be there for only a few years. This is a personal choice, and so it will vary a lot. However, university degrees are at least partly about educating the student, so they can't expect you to know everything already.

Finally, admissions processes vary wildly across departments, universities, etc., so it really depends. In my limited experience based on watching others at one US university, master's admissions tend to be looser than PhD admissions in the sense that master's students who can pay are frequently admitted without a supervisor lined up under the assumption that they will find someone to supervise their thesis while doing the required coursework during the first year. PhD students usually aren't admitted directly to a PhD program unless some professor puts up their hand and volunteers to take them on. In other countries, I understand that students apply directly to a potential advisor and as long as they meet minimal criteria, they will be admitted to the university. Many US programs may be de facto like this but separate the university/departmental requirements from the professors' requirements on officially. 

Finally, it's my understanding that many students are admitted to master's programs with the implicit expectation that they will transition to the PhD program over time after they find an advisor.