Spend more time in your admin panel...

For your first question:
ADMIN >>> CONFIGURATION >>> MAXIMUM VALUES >>> New Product Listing - Limited to ...

For your second question:
There is NO SEO ADVANTAGE in URL re-writing in zencart. In fact, you can do more harm than good. Re-written URL's are not necessary for zencart, and there is no evidence that they improve rankings. If you are intent on doing this, then there is only one module that is reasonably safe and reliable and that is CEON's URI MAPPING module. DO NOT USE ANY OTHER MODULE... you could wreck your site's link navigability.

For your third question:
Complex shipping is not easy to configure, so again, look at CEON'S ADVANCED SHIPPER MODULE (http://dev.ceon.net).