It's typically over a million, and in this kind of use it's
going be FAR more reliable than a hard drive would be in
both shock resistance and general lifespan. You incur no
limitation for reading from it though.
The more significant factor will probably be the proprietary
lithium battery pack, if no other part suffers some random
mishap then it'll make the player near useless after 3 years
or so... while the flash memory is likely to be good for
another dozen or more.