How to limit the number of nodes created using the Feeds module?
You'll also need to clear your caches after setting $conf['feeds_process_limit'] = 50;.

Alternatively, if you are just looking for a quick limiter (during development of a custom importer, for example) this trick worked for me: 
Limit number of feed items to import ($conf['feeds_process_limit'] not working) (an active issue in the Feeds issue queue).