It is hard to get reliable information regarding how many people choose TEFL as a job every year. If you try and 'Google' it, the first few results are inevitably provided by the very organisations who are trying to sell you their Certification courses. More on those a bit later. According to the British Council, about 4 million people globally take the IELTS examination, and TOEFL has been taken by more than 40 million people since it was created in 1962. These figures do suggest that a lot of people are learning English, and consequently the demand for teachers remains high, one would surmise. However, just because the world seems to need a lot of teachers of English does not mean anyone can simply transform themselves into a professional teacher . . . or does it? Read on. . . 

It depends: whilst it is true that you can find teaching jobs in some countries and organisations without any kind of qualification, you need to seriously think about what kind of outfit would do that. There are plenty of, shall we say, not terribly reputable online providers who still hire people just because they are 'native speakers' (don't get me started!). I think you should also ask yourself how credible you would be if you were hired to teach having not received any kind of training in language or methodology. Maybe I am a bit biased . . .  I would urge you to get some training before being let loose on some unsuspecting learners of English. 

I don't think it is unreasonable to answer, 'no'. I mean, there are so many qualifications out there (probably because of the demand?) and some are bound to be better than others. Whilst I really don’t want to single any qualifications out for praise or criticism, what I will say is that some courses (the better ones, to be clear) include observed teaching practice and are accredited by reputable organisations (not by themselves, be careful!). I encourage you to do some research and make sure you complete a course that is recognised by large respectable employers, such as the British Council, International House or universities rather than certain outfits which promise you employment on completing their courses: these are often self-accredited and only recognised by themselves.