How often do universities in General have admissions for graduate programs? I am thinking to apply to a graduate program in computer science (Research Masters or PhD), but I am not sure how often universities accept applications.

For example I know the University of Toronto does so each December (so only once a year). But I wonder if other places (anywhere in NA or Europe) have different patterns.
In the US there is no general system. Most universities will have deadlines for admission once or twice a year with a few having more. They are matched to the normal Fall and Spring semester system used in most (not all) universities. However, most students enter in the Fall with deadlines for application likely sometime in the Summer or possibly the Spring. 

But each university will operate on its own schedule so you need to check. 

Another consideration is when you need to apply for positions like Teaching or Research Assistant. Those might possibly be once a year even if the admission deadlines are twice. So, check about that also. Likewise scholarship and grant funding can have separate schedules. 

Narrow down your search to a few universities and look at the details for those.