how do I limit the amount of widgets that someone can use/add?
Ultimately, you can't. That's for the user to decide. The best you can do is to try and ensure that the horizontal block wraps over multiple lines. So - ok - it might look awful but if all of the text/links are readable and accessible, then I'd argue that, as a designer, you've done all you can do. At the end of the day, you cannot idiot-proof a theme.
how do I put in defaults if the blog owner doesn't select them
Place default content in there - e.g,:
<?php if (!function_exists('dynamic_sidebar') || !dynamic_sidebar('Top Tabs') ) : ?>
<?php endif; ?>
or if they only select say, two instead of three
Widget default behaviour is to replace all default content as soon as a single widget is added. So if your design holds up with a single widget in place, again, you've done all you can do.