Is it usual that the chance for a PhD student that aims to apply for the second semester of the educational year (i.e. spring semester) and sends his/her application for professors may be lesser than who want to apply via connection with professors for the first semester in an educational year (i.e. autumn semester)? In other words, does volumes of accepted PhD students in the autumn semester influence on reduction of acceptance rate and opportunity for a Prospective PhD student who aims to apply in the next spring semester or each semester has its own capacity for accepting PhD students?
In the UK, most PhD studentships for home (UK/EU) will be advertised in the autumn, and also those few places that have PhD programms will have closing dates just after Christmas. In most cases this will be to recruit to programs that will start in the autumn of the following year.
This is not a hard and fast rule, and plenty of people will get positions later in the year, although many of them will still start at the start of the academic year in October.
This is not to say that no one starts inbetween, its just much less usual, and often suggests an unorthodox funding route, such as self-funding, foreign government fellowship or industrial sponsorship.