I wonder if it is OK to have e.g. 20 or even 50 subdomains on one hosting account on one domain? - I mean if it can't cause problems to the server of my hosting provider? I want to build many sites on different subjects and don't want to spend much money on domains, so I'm going to create many subdomains.
Answer depend on your hosting service. If you are hosted by reputable services such as GoDaddy, Hostgator and the like, there is not problem and it's allowed do such thing with the note of caution regarding avoidance using resource-intensive scripts. If you are hosted elsewhere, double check what your package allows or ask support when in doubt.