Normal CD's are .CDA files which as you say are about 20 per CD(R).
.MP3 should be burnt as Data CD's and I would expect loads more on the CDR. (Up to 100 ?)
As above, burn at the slowest speed possible (1x to 4x) and go for quality makes of CDR.
I like Verbs, Imation, TDK, etc.
Also try to avoid paper type labels, if they come unstuck at the edges then it can damage the unit and fail to eject them properly. Labels also add to the CDR depth and that can cause problems.
A good source of quality CDR's is
http://svp.co.uk/products-list.php?cid=68