How do I get my Mp3 Player to store up to 500 songs?

I have a Sansa c240 1GB Mp3 player. Last night I converted all my wma files into Mp3 until the back of the box said it can store up to 500 wma and 250 mp3 files. When I converted all my songs which took forever I only could fit 206 mp3 files on to it, but before I had 233 mp3 and wma files. How can I get a higher number than that?