There are never too many to redirect, no matter how many there are. 301 redirecting your old URLs is absolutely critical. Without those, no matter what you do, you will never reclaim any external link value pointed to those pages. It may not amount to much on a per-page basis, but added together it's probably pretty significant.

If the old URLs followed a consistent pattern and the new URLs do as well, a good programmer should be able to create global redirects on a site-wide level rather than having to do it on a page by page basis.