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.
As long all of your websites not hogging the server resources and still within the limit the hosting company gave you, you're free to create as many subdomain that they allocated to your hosting account. - Dynashox -
Yes, I do not think your hosting provider have any problem if you have limit for your account to create sub domains than you can easily make your website on your sub domain. But your website should not match each other if matched than google will penalty you on your search ranking.
It depends on your hosting provider, the OS they use, and how sub-domains are set-up. In our windows systems, subdomains are actually completely independant sites within IIS (the web server). The root of the subdomain is a folder within the main site, but the actual subdomain is an independant web site in the web server (just like a domain). Just like any domain, it uses a little bit of system resources just because it exists, and someone with 50 subdomains uses as many system resources as a user with 50 domains. OK, they might not require much traffic or CPU for processing, but in terms of resources used just to exist they use as much as a stand-alone domain.