It ultimately depends on the file size which basically varies with the length and recorded quality of the MP3 files.
For example (I'm just making up these numbers), if you record a 5 minute song @ 128-bps (bits per second), then it's file size could be 5MB. Recoding at 256-bps will result in a 10MB MP3 file. Lastly, recording at 64-bps will give you a 2.5MB MP3 file.
Based on the above made up file sizes, a 700MB CD can hold:
a. Roughly 280 5-minute songs @ 64bps. Will sound crappy.
b. Roughly 140 5-minute songs @ 128bps. I think this is average.
c. Roughly 70 5-minute songs @ 256bps. Slightly better than average sound.
For the record, I do not like encoding my music to MP3, they just sound like crap regardless of the bit rate used. I've used OGG Vorbis in the past instead and have switched over to the FLAC format not too long ago.