RSS is just another way of expressing the data. It depends entirely on the site. If the site provides a way for you to specify how many items you want (which is unlikely), then you should know that that won't work on other sites.
Technically speaking, formatting the data in RSS is no different than formatting it in HTML. For example, many sites (including this one), need to represent some sequential data (questions in SO's case) on a page in HTML. To do this, the site will iterate through some data source (like a database), and output HTML so your web browser can render it, until it hits some limit. Knowing that limit is impossible, as it depends on the site. This is exactly what RSS does: it iterates through a data source, spitting out XML as it goes along. Again, knowing the limit is not possible.
Is this something that is only possible from the other end ...? In which case, how is this achieved?
If you can change how your site generates the RSS, simply remove the limit. I know this is vague, but it really depends on the implementation. There are dozens of RSS implementations, all different, and all behaving differently.
So my point is, nothing will work universally, you have to change the site itself to modify that behavior.