When we talk about "how many audio tracks," we generally don't differentiate between recording versus playback. You can record and/or playback any number of tracks up to the limits specified. So, at 192 kHz, you can record 24 simultaneous tracks, or playback 24 simultaneous tracks, or record 12 while playing back 12, etc. Any combination up to 24 tracks.24 tracks at 176.4/192 kHz is currently the limit, regardless of what kind of computer you're using. If the HD cards are in a chassis, the limit is "officially" 16, but you may be able to get 20-21 tracks to work reliably (your mileage will vary).At 88.2/96 kHz, the maximum number of tracks for an HD1 system is 32. With HD2 or HD3 systems (and beyond), you can use 64 tracks at 88.2/96 kHz.At 44.1/48 kHz, the maximum number of tracks for an HD1 system is 64. With HD2 or HD3 systems (and beyond), you can use 128 tracks at 44.1/48 kHz.The above information pertains to PCI performance limitations. To achieve these track counts, you'll also need to have enough (fast) disks. For high-density edits across lots of tracks (~ 200 msec per edit/crossfade), you'll be better off using around 16 tracks/disk at 44.1/48 kHz. At 88.2/96 kHz, 8 tracks/disk. At 176.4/192 kHz, 4 tracks/disk. Again, these are for the most intensive high-density edit applications.If your tracks don't have that many edits, then you'll likely get many more tracks per disk. With the fast SCSI and IDE drives these days, it's possible to get 48 tracks (or more) per disk at 44.1/48 kHz. It really depends on the nature of your edits and how fast your drives are. Also, defragmented drives (freshly formatted) work much better.