The speed of a hard disk drive determines the number of tracks that can be played simultaneously.
The rotational speed is the most important factor. Disks of 7200 rpm are good for audio, 10,000 rpm even better.
The technology used to interface the hard disk drive to the computer or other recording system is also significant.
SCSI is the fastest, ATA and Firewire are not yet as fast, although they are usable for up to 24 simultaneous tracks.
For the greatest track count, multiple disks are often used.
There are certain factors that limit track count:
Great home recording starts with a great home recording studio. It doesn't need to be expensive if you know how to select the right equipment for your needs.