It doesn't 'stop' at 48.
That is as many entires as it can display at that screen resolution.
To access something further down the list, with the focus on the list, type in a start value - say 5 and it'll jump down to say 50, now use the up/down arrow keys to scroll through the list...
This is a common issue with the 'inputbox'.
The only way to get a scrollable list is to use a webdialog of your own devising...