When you created the widget from the block, it asks for a wiki page to use as the scope. Make sure you leave this blank, or it will only show Latest Articles that are children of that wiki page. If the wiki page can't have children, then the widget would be empty.
There is no cutoff. It will always show the 50 most recent results. If you scroll to the end, you can load another 50, and so on.
However, I have noticed that the option to change the activity filter doesn't have any effect on the initial loading of the widget (which completely cripples the ability to use it as a Latest Articles block for now, although that was the original intent). This bug appeared in 4.0.0 RC 5. Since 4.0.0 RC 5, it would always show ALL activity on initialization, regardless of the default filter. I have fixed this for the next release.
In the next release, I have also made it hide the filter selector if there is only 1 filter available.