For the long distance lines, there are 23 "Radiales" and 23 "Transversales." If you like maps (what traveller doesn't?), you could view them all by pulling up one, e.g.,
http://www.renfe.es/empresa/glineas/t14.htm (the "Picasso") and replacing t14 with t1, t2....t23 for tranversales, and lr1, lr2...lr23 for the radiales. Shows you easily what long distance trains there are. Tranversales cross the country, radiales radiate from Madrid mostly.