I will say that you should worry about traffic and not PageRank with the change you intend to make.
If you implement the 301 redirect correctly from old URLs to new, you should be on the same traffic levels within a month (maximum) after this change. However, the double check if the redirect are working properly from the time of switch.
There is no limit to number of 301 redirect that you can add however, avoid and chain redirect like
Page A >> Page B >> Page C
Only issue that you might face is increased server latency with so many redirects in the htaccass (if you are using this method to redirect) as for every request this file will be scanned for a possible redirect. An alternate can be that you keep the old pages on the site and do a php script 301 redirect at page level which will be time taking as you might have to manually update the code of each page. Note that the script should be at the very beginning of the page so Google crawls it first and moves on. With this method you can ensure that page speed is not affected for the pages where no redirects are added.