As the super mod says, but I just want to stress a point he was making a little clearer for some.
If you are putting music to play in standalone regular music CDs (ex: Music CD you buy at HMV or other music stores) then the determining factor for how many you can fit on the CD is the length of the total songs. If all 18 songs are no longer than 80min or however long your CD-R is rated for, then no problem, otherwise no can do.
Quality in bitrate has no factor as it will still be the same length, only with worse sound lol. With MP3 CDs you can fit as many as determined by the size of the files (in most cases CD-Rs come in 650MB-790MB).
lol, I don't know if I made it clearer or more complicating lol