It depends on the company but the majority only selects the "best" from the previous stages for the final stage. You get to the final stage and out of say 200 applicants there may only be about a handful who have made it. Of course, this number varies by company or the quality of the candidates.
For large corporations, there may be several assessment centres where candidates are allocated and distributed among those different slots. Most of the time there's just one assessment centre so that's essentially an all vs all kind of battle for only a limited number of spaces in the company.
Either way, competition will be tough but it's still always good to turn up unless you're sick or you genuinely hate the role. Assessment centres aren't really as daunting as they seem, I've been to a lot in my time finding a placement and most of them have been fun.