I'm ripping my CDs to disk I would like to stack them up on other CDs
so I can have as much music on one CD as I can
what ever format I use it has to work with my 10 year old cd player
anybody else done this ?
what works best

You don't say whether you're burning them to CD as MP3/OGG/other-digital-format, or whether you're doing it as a standard audio CD. From the context, I'm assuming it's as audio files.

CD Audio files are quite large, compared to MP3's/etc. I'd say probably 10-20 tracks (depending on song length), is probably the most you're going to get. As far as I know, audio CD's are only in one format...most burning programs will ask if you want to burn it as an audio-only CD.

If you're talking about burning data, what format you use is less important that what kind of data and the bitrate at which you rip the CD. A 128Kb file is smaller than a 256, but doesn't sound as good. But, smaller file=more data on disc.

yes it will be in audio format (I want to be able to play the new CDs on a simple 10 year old player )

If you're ripping your CD's and need to keep in an Audio CD format, probably the best format to keep them in is WAV format. Then you can create the final CD's using from the WAV files. If you're goal is to try and compress these to reburn to fit on less CD's, you'll find that you may be just wasting your time and CD's, might as well just keep the original CD. Like mentioned, you'll probably only fit at most 20-30 songs on each CD, depending on length. Basically taking this approach, you're just making a bunch of backup CD's for each CD you own really. To save the amount of CD's used, upgrade your simple CD player to one that supports MP3, OGG or another more compressed format, then rip and burn in new format. I think I once fit about 130 or so songs on one CD-R once myself.