The stock email application and many other I've tested present a list of 25 emails per page.
The user can however load older emails on demand. This is done due to two main reasons:
- to prevent large transfers of data that end up using the device communication thus leading to a faster battery drain;
- to preserve memory consumption that in turn would leave your device slow and inefficient.
I haven't found any option on all the applications I've tested that allows the user to define how much emails should be presented each time.
I did find that the Gmail app auto-loads a new emails list when we reach the scroll limit, thus kinda facilitating the users trouble, but even with this scenario, the user needs to scroll all the way down to get the new list, and yet again, the Gmail app like the others, doesn't have a setting to define the number of emails to be loaded each time.
Essentially, this is a status-by-design conception used by Android and app developers. You may come across with an app that allows you to go around this issue, unlikely it may be, but for the stock app, the reason as to why it loads only 25 emails at a time are the ones explained that sum up as performance issues.